Harry Potter and the Chamber of Secrets is the second book in the Harry Potter series. As J.K. Rowling has told us, the character of Harry Potter "really is the whole story. The whole plot is contained in Harry Potter; his past, present, and future" (source). So what would any of his books be called but Harry Potter and the Something or Other?
As for Chamber of Secrets, the Chamber is a major plot point in the story. It also sounds intriguing: if you know that someone has a secret, don't you want to hear about it? And a whole secret chamber – that's really alluring. So, if you don't want to read the novel just because it has Harry Potter in the title, maybe you'll buy it because Chamber of Secrets sounds mysterious and interesting.
The last chapter of Harry Potter and the Chamber of Secrets ties up all the loose ends of the novel. Ginny is reunited safely with the Weasley family. Immensely annoying Professor Lockhart leaves his job at Hogwarts because his memory has been permanently damaged (thanks to Ron's faulty wand). Harry realizes that Lucius Malfoy is the one who slipped Riddle's diary to Ginny during the scuffle at Flourish and Blotts at the beginning of the novel. Harry uses the diary as a way to force Lucius to free Dobby, his house-elf. So Dobby is no longer tied to a Dark wizarding family. At the celebration feast the evening of all of this adventure, Hermione and all the other Petrified students return to school. All seems (mostly) right with the world.
The key point about the end of Chamber of Secrets is that Professor Dumbledore has a chance to emphasize the importance of choice in determining who you are. Both Harry and Tom Riddle share a lot of characteristics: a lonely childhood as an orphan wizard raised by Muggles, lots of talent, and even Parseltongue (the ability to talk to snakes). But Riddle responded to his miserable youth by becoming Lord Voldemort. Harry has chosen time and time again to stay loyal to his friends and his principles. Even though Voldemort has marked Harry with some of his own powers, Harry can still use those traditionally Slytherin talents for Gryffindor-ish purposes.
The thing that's odd about this conclusion is that, yes, Harry gets to choose to be in Gryffindor. He chooses to stand on the side of good, which makes him the boy hero we know and love. Later on in the books, there are all kinds of constraints on Harry's choices, what with Voldemort's manipulations and various prophecies. So the issue of free choice versus fate that seems so firmly decided by the end of Book 2 actually grows more ambiguous as the novels continue. Chamber of Secrets still presents a relatively black-and-white view of Good triumphing over Evil. This stark morality starts to fall apart as we get to Books 5 and onward, when the way forward for Harry seems less and less clear.
This sharply black-and-white depiction of morality is what makes Chamber of Secrets seem more like a kid's book than the later, harder books in the series. Once Voldemort rises at the end of Goblet of Fire, we get major multi-book plots involving the war effort against Voldemort, Snape's secrets, and the escaped Death Eaters.
By contrast, at the end of Book 2, we still know that Voldemort is somewhere out there and that there are Dark forces threatening Harry. Those forces, though, remain vague and unknown. Harry has won a decisive battle against them. He hasn't yet suffered the kinds of losses that mark Books 4, 5, and 6. That's one reason why Chamber of Secrets sometimes feels more pleasurable to read than, for example, Harry Potter and the Order of the Phoenix: Harry's decisions are easy and satisfying, and he wins everything in the end. The complexity of the later books makes them more mature and morally interesting, but they also become less total fun than Books 1 through 3.
The setting that we really care about in Harry Potter is, of course, Hogwarts School for Witchcraft and Wizardry. Privet Drive is simply nightmarishly normal and Mugglish. There’s nothing fantastic about the Dursleys' home at all.
By contrast, Hogwarts Castle – which is only accessible through a magic train we Muggles can’t even see – is filled to the brim with giant squid, trick staircases, Hungarian Horntails, and Forbidden Forests. Every corner of the castle seems to have some new enchantment to discover. It’s like a dream come true for everyone who has ever been bored with humdrum Muggle life or school.
But along with the living portraits and Quidditch pitches comes real danger and emotional agony. Just because the magic world is cool to read about doesn’t mean that it would be safe or pleasant to live there. A lot of social problems that we endure in our lives persist in the wizarding world, including poverty, racism, and terrorism. J.K. Rowling sums up:
Harry entered this world that a lot of us would fantasize would be wonderful: “I’ve got a magic wand and everything will be fabulous” – and the point being that human nature is human nature, whatever special power and talents you have [… Harry] walks into this amazing world, and it is amazing, and he immediately encounters all the problems you think he would have left behind and they are in an even more extravagant form because everything is exacerbated by magic. (source)
When Fawkes the phoenix brings Harry the Sorting Hat in the Chamber of Secrets, the Hat produces a ruby-covered sword for Harry. Harry uses that sword to kill Slytherin's basilisk. The sword originally belonged to Godric Gryffindor. Harry's ability to draw it from the Hat to kill Slytherin's monster represents a fairly straightforward confrontation between good versus evil: Harry is on the side of one of the good founders of Hogwarts, not the pureblood-preferring bigoted one who eventually left the school.
At least, that's the black-and-white Slytherin-versus-Gryffindor story we're getting early on in the Harry Potter series. This equation of Gryffindor = good, Slytherin = bad becomes more problematic with the introduction of Peter Pettigrew in Book 3 and, of course, the revelation about another character (we don't want to spoil it!) in Book 7.
Tom Riddle's diary is not so much a symbol as a plot point. The diary gives J.K. Rowling an opportunity to bring Harry face to face with the young Voldemort, to start recognizing the similarities between the two, and to affirm his wish to walk a different path. What we like about the diary as a tool to achieve these goals is the way she presents this magical object.
Riddle describes the book as "a memory [...] Preserved in a diary for fifty years" (17.17). Yet he also talks about his possession of Ginny as an exchange of soul:
"So Ginny poured out her soul to me, and her soul happened to be exactly what I wanted [...] I grew powerful [...] Powerful enough to start feeding Miss Weasley a few of my secrets, to start pouring a little of my soul back into her." (17.45)
All of this talk of souls preserved in objects reminds us of something: the Horcruxes of Books 6 and 7. Even though Riddle's diary has a useful plot function in Chamber of Secrets, J.K. Rowling also uses it skillfully to foreshadow future developments about the state of Voldemort's soul and the secret to his continued life after getting hit by a reflected Killing Curse years before. We may not know that the diary is called a Horcrux by Book 2, but we do know that Voldemort has been leaving bits of his soul around – an important point to bear in mind as the series continues.
The Sorting Hat places each new generation of Hogwarts first-year students into the four Hogwarts Houses. The Sorting Hat moves through your mind to decide if your primary characteristics are courage (Gryffindor), ambition (Slytherin), hard work and fair play (Hufflepuff), or intellect (Ravenclaw). So the Sorting Hat represents one quick way of characterizing all the Hogwarts students we ever meet: if you're a Ravenclaw, you're smart, if you're a Hufflepuff, you're patient, etc.
When an interviewer asked Rowling point blank, "Has the Sorting Hat ever been wrong?" Rowling said simply, "No" (source). We sort of can't imagine how that's possible – don't people ever change or outgrow their houses? – but we guess that's the point: it's magic.
J.K. Rowling has also confirmed that, when the Sorting Hat speaks to Hogwarts students, its words come "from the founders themselves" (source). So the Sorting Hat is the voice of Hogwarts; it is intimately tied to Hogwarts' history and origins. At the same time, the fact that the Sorting Hat is never wrong about people troubles us a bit, since weren't the founders of Hogwarts ever wrong? What about that whole Slytherin-versus-all-the-other-founders argument? Doesn't that cast doubt on the people who created the Sorting Hat in the first place?