First, note that these novels haven't been licensed in the US, so you have only two options for reading them -- (1) learn Japanese and import them from Amazon.co.jp, or (2) read the bootleg translations from Baka Tsuki. I opted for the latter, and while the translations are kinda rough in places ("Where's the delinquent Takasu!? Come and help us.") they're entirely readable, to the point that I managed to devour volumes 5 and 6 in a single day (roughly equivalent to reading a 500 page book in one sitting).
The plot is a straight-forward rom-com set-up: Taiga and Ryuuji are high school students in love with the other's best friend. Upon figuring this out, they decide to help each other out, but in the process end up falling in love with each other. There are lots of incidents along the way, most of them standard tropes for a Japanese school series (summer vacation, the class trip, the school festival).
But what elevates this series is the characters. Upon first introduction, everyone seems to fit into standard archetypes that should be familiar to anyone who's watched even a little anime: Ryuuji is the Ordinary High School Student; Kitamura the Class Representative; Taiga the tsundere; Ami the Libby; Minori the Cloudcuckoolander. But Yuyuko Takemiya isn't content to let her characters languish as stock figures, but instead uses the story as a psychological study to examine what such characters would be like in real life.
Take Taiga. We first encounter her when she decks Ryuuji for simply bumping into her in the hallway. A little later she nearly beats him to death in order to retrieve a love letter she stuck in his backpack by mistake. Even once they become friends, she continually refers to him as a "dog" and slaps him around if he gets out of line. All of which is typical tsundere behavior, and most authors would treat it as simply humorous. And as with a tsundere, Takemiya slowly reveals that this attitude is armor that shields Taiga's soft, gooey interior, but she then proceeds to ask the question that other authors ignore -- what sort of screwed up circumstances would lead a girl to act this way.
The answer is that Taiga has severe abandonment issues: her father kicked her out of the house for not getting along with her step-mom and set her up in a luxury condominium with a large monthly stipend, and her natural mother wants nothing to do with her. When we finally meet Taiga's father, he turns out to be one of the few totally despicable characters in the series. His explanation for why he abandoned Taiga is absolutely gut-wrenching -- she's blood, so she has to love him whatever he does, while his wife requires effort to keep around. Can you blame her for being a bitch?
Ryuuji has the inverse problem. His mother ran away from home when she became pregnant with him in high school. She now works as a hostess in a businessmen's bar (i.e., she's paid to flirt with customers and get them to buy lots of drinks), and comes home every morning so drunk she's lucky if she makes it to her bed before passing out. All the burdens of taking care of her and maintaining the household -- cooking, cleaning, shopping -- fall upon Ryuuji. And he does it gladly, because he's a good boy. And when he sees the state in which Taiga lives, he instinctively begins taking care of her as well, going to extraordinary lengths to make her life better. Because he's a good boy. When he finds out Ami has a stalker, he immediately offers to help her. Because he's a good boy. When his friend Kitamura faces a personal crisis, Ryuuji tries to help him. Because he's a good boy. As the story progresses, Ryuuji takes on more and more burdens to help others, often at the cost of his own goals, until in the final three books the weight becomes too much and he begins cracking under all the pressure.
But by far the most fascinating character in the series is Ami, a famous teen model who's been forced to take a break from her career because of a stalker. Her genuine personality is acidic yet strangely likeable, but as a teenager working in a world of adults, she's cocooned herself with a fake persona, Ami-chan, a cute, sweet airhead. When she returns to the world of her peers, she no longer knows how to act, and for the first few books she's the closest thing the story has to an antagonist as she takes a dislike to Taiga and tries to get at her through Ryuuji (though it's never clear, even to her, whether she's doing it out of spite or if she has feelings for Ryuuji). But her real problem is, having spent so much time around adults, she understands nuances that the other characters are still discovering, giving her an uncanny ability to understand what's going on around her. Yet her attempts to warn Ryuuji are either ignored or end with someone getting hurt. The more she tries, the worse things get and the more she loathes herself.
Minori is an enigmatic character, seeming to be aloof and wacky, yet we get occasional glimpses of the depths she's hiding, such as in the fourth volume when Ryuuji finally gets up the nerve to ask if she has a boyfriend, and she replies with a seeming non-sequitur about ghosts and UFOs. But as she goes on for several pages on the subject, we begin to see that she's actually using a spectacularly extended metaphor to explain her feelings on love. But these revelations about Minori's inner mind are sporadic, and she remains a puzzle far longer than any other character. Even the ever-insightful Ami doesn't fully comprehend the extent of Minori's mask until the end of book 8, and when she finally figures it out she is astounded:
"Amin, why are you mentioning this?"
Hearing this question, Ami's response was: "Because you've always been playing dumb. I. Think. You. Are. Truly. Amazing. You can just pretend as though nothing has happened, say whatever suits you just to get through the occasion.... You really know how to act like a goody-goody."
"What's that supposed to mean?"
"I wonder, what was that supposed to mean?"
By contrast, Kitamura is a simple character. Although he too wears a mask (if you can't tell, the mask is a central theme to this series), it's more straightforward than the others -- at school, where he's the student council vice president, class representative, and captain of the softball team, he's serious and businesslike, but get him away and he becomes a wild-and-crazy guy with an penchant for exhibitionism. He does have a reason for this dichotomy, and once that reason is removed he relaxes his mask at school.
Honestly, these are some of my favorite characters in all literature. I would place Ryuuji, Minori and Kitamura at the same level as Hermione Granger and Luna Lovegood, while Taiga and Ami come close to toppling Samwise Gamgee from the top spot.
The other thing I love about this series is that there are no significant villains. Sure, Ami's antagonistic when she first appears, but it's not because she's a bad person -- and in truth, it's Taiga who's the biggest obstacle to peace between them. There's also Ami's stalker, but he's little more than a plot device to bring the characters together. Even the one significant fight in the series, Taiga vs Kano Sumire, begins with Taiga thinking,
Was this the wrong thing to do? It could be the wrong thing to do, but Taiga didn't know.
She only knew that she wouldn't stop.
She was already unable to stop.
The only character who is an out-and-out bad guy is Taiga's father, and he only appears in a single book.
Hegel once observed that the truest form of tragedy is when two equally correct but mutually exclusive ideals come into conflict. Though this series isn't tragic, Takemiya takes a similar approach to create a situation where no matter what anyone does, someone will get hurt. What keeps the story going is that these characters are all good people, and when they're presented with the choice of hurting someone else or getting hurt, they choose to take the pain upon themselves. The whole series could've ended by book 7 if even one character decided to act selfishly.
But despite the lack of villains, Takemiya still finds a way to end each book with a cathartic climax (except #4, which is a quiet character study with an insignificant climax followed by a satisfying denouement). Most of these are major heart-breakers, such as the revelation of how big a jerk Taiga's father is in book 5, Taiga deciding to take down Kano Sumire in book 6...
...and her near-death experience in book 8. But the best of them is book 7, which is like getting kicked in the gut repeatedly by Malcolm McDowell. The anime adaptation is on a level with the dog episode of Futurama and that's heavily watered down compared to the novel.
And speaking of the anime, if you enjoyed it you should definitely read the books. The series did a good job of showing all the main points in the story, but there are lots of great details that had to be discarded, such as Minori teaching Ryuuji to ski, or the full extent of Taiga's plan to win the swimming contest. The worst is the treatment of Koigakubo-sensei -- in the novels, she's an unbelievably excellent teacher who just happens to have a horrible social life, while the anime turns her into a complete butt-monkey whose entire raison d'etre is to find a man and get married. Ami suffers to a lesser extent in the last third of the series as the last part of her character arc (in which she comes to hate Ryuuji and herself for the trouble they've caused) is abandoned in favor of making her another harem girl. (Although I do have to credit the anime writers for giving her the best line of the series: "Talking to you is like having my period.")
The anime does improve on one thing from the novels -- Taiga's delirious speech at the end of book 8 rambles on far too long after revealing the key piece of information, while the anime boils it down to the simple but effective declaration:
None of this should be taken as a slam against the anime -- it truly is one of the best series ever done, it just isn't as good as the novels, which deserve proper American releases with a good marketing department behind them so they're read by people besides geeks.
 Tsundere is a Japanese term that can be roughly translated as "bitch bitch, lovey dovey" -- basically a character that behaves like Princess Leia does around Han Solo.