I feel it's not cheating to put a series on here instead of a single book (most really well-written series are like one really long book anyway), but I have so many books in my inventory that I really can't even limit myself to just one series. The first five or six Honor Harrington books (up to In Enemy Hands which is the last really good one) by David Weber; C. S. Forester's Horatio Hornblower series, on which they were vaguely based; both The Enchanted Forest Chronicles and the Lyra series by Patricia C. Wrede; Blitzcat by Robert Westall (that'll probably be the only thing with no relationship to fantasy and sci fi on my list); and a couple of books none of you have probably heard of: Emerald House Rising by Peg Kerr and Count Scar by C. Dale Brittain (yes, you heard me, Count Scar, not the Yurt series, however much I may love it!).
Quite frankly, if I went and looked at my bookshelf, I would probably find even more books that I can just pick up and spend a lovely Sunday afternoon with, no matter how many times I've read them before. It may sound cliché, but many of them are my old friends, some of which I had to make a special effort to seek out when I grew up and moved out on my own, away from my childhood library. Given that I always read at a level significantly higher than my grade level, even relatively dense things like Larry Niven, Anne Rice, and Stephen King are included in those pleasant childhood memories.