I'll second Don Quixote... it's a helluva book to get through (length-wise), but really terrific. The humor and commentary on the state of things when written is incredible.
If you'd like a lighter option, I have to strongly, strongly suggest the book I'm currently on, A Slip of the Keyboard by Terry Pratchett, which is a collection of articles and speeches from Pratchett, spanning his entire career. He compiled them, and wrote a note a bit of context to precede each; it was published about a year before his death. Full of wit and laughs, insights on being a writer, commentary on fantasy and sci-fi literature, and insights on philosophy and religion (especially from his very late career, knowing he was approaching death and doing so with very open eyes).