I have three books which get reread on a reasonably regular basis (every few years), each for slightly different reasons.
Conagher by Louis L'Amour, which keeps me going back to it so I can get under the skin of what makes it work. It's a slow burning western about the very tentative romance which blossoms between a worn out old cow hand, little more than a hobo who owns a horse in truth, and a lonely widow. There's a sub-plot about a feud and a gun battle near the end, but these feel more tacked on to appease L'Amour's core readership at the time it was written. It falls into the cracks between what it wants to be and what it's expected to be. The book shouldn't work, but it does and I want to know how and why.
Ravenheart, by David Gemmell. Another book which pretends to be one thing, but is really something else entirely. It's nominally an adventure about a brewing revolution in an alternate version of Scotland; under the surface, though, is a story about love, redemption, loss and self sacrifice and it always makes me cry. That's probably why I go back to it. For the gentle portrayal of people letting their pride get in the way of their own happiness and how they redeem themselves for those mistakes. At heart, I'm a bit of a big softy.
The Lost World by Sir Arthur Conan Doyle, because it's a rip-roaring adventure story that barrels along at a tremendous pace, filled with action, adventure, intrigue and big honking dinosaurs chasing people. I can be all sensitive and emotional, but sometimes I just want me some dinosaurs, dammit! I'll also admit that it was the first adventure novel I ever read, when I was about nine years old I think, and it has the literary equivalence of comfort food for me. When I'm miserable, ill or both, I'll dig out the Doyle and go back to the Plateau.