I don't know about you, but I always get tickled like anything when I encounter a book that's just an extended rant about, well, about whatever bee the author has in his or her bonnet, and the actual fictional part of it is just an excuse for the character to rant. Oh, there are books/authors (Orson Scott Card's later works come to mind) where the author, or some authorial insert character, goes on some brief screed about something the author would have said were he writing an essay, but I'm talking about where the book is basically the author's argument for his or her philosophy.
The author who has basically become the byword for this, as far as I'm concerned, is Ayn Rand. I actually think Rand is a rather fun author when she remembers that she's, y'know, writing a book, and isn't getting caught up in trying to get her characters to Declaim All About How Collectivism Is Bad. I actually think The Fountainhead is a great bromance-dysfunctional-romance book, very readable and entertaining (if highly dysfunctional -- seriously, the misogyny of coming up with a character like Dominique Francon would make me think Ayn Rand wasn't female if I didn't know better), when she's not lecturing. Atlas Shrugged, of course, is all about the lecture and not about the characters at all, and I find it all but unreadable. But I laugh a lot to myself when reading her books. And roll my eyes a lot.
I recently read Little Brother (Cory Doctorow), the second Doctorow I've read; I get a similar feeling from his books (although he, at least, doesn't have his characters go on thirty-page rants). I enjoy his rants too: don't let the government (or other centralized institution) have power over you! Information wants to be free! The book is clearly an excuse to Preach The Gospel of Anti-Authoritarism; the characters, while not quite as one-note as Rand's, are still, um. Primarily important for how they feel about The Man?
Oh, and there's Unwind (Shusterman), which is All About How You Should Be Pro-Choice, Dummy. Because apparently the alternative is killing 16-year-old kids! Like Rand and Doctorow, Shusterman is a good writer, I think. But wow, the anvils!
Are there other extended fictional rants you know about that you do or don't enjoy?