There's lots of good advice already. The only thing I would add, is to keep the books upright on a shelf (just like libraries and bookstores do).
I used to try and keep all my RPG books in a pristine condition.
Then, I went to the first Chicago Gameday (way back in January of 2002) and played in cdsaint's
Weird Wars game. My copy of the book was nearly flawless. His copy looked pretty beat up (the game had only been out for five months).
And in that moment, I realized that I didn't care what my books look like. I want to
use them. And cdsaint's book said (to me) that he loves that game and has gotten countless hours of enjoyment out of playing it.
I try to keep them in good shape, but if that doesn't happen, so be it.
I love my (mostly) pristine books. They look very nice on the shelf. But if the books I use a lot show some wear and tear, well, great! That means I'm using them and really getting my money's worth.
Now, the coke spilling--that is UNFORGIVEABLE! If I ever did that, I would offer to swap my (better) copy or buy the victim a new one. I recently bought the "Serpent Kingdoms" book. While carrying the book from the car to our house, my son spilled his hot chocolate all over it. I was upset for about a minute, and then realized that it's just a book and there are more important things in life--like creating Serpent Kingdoms NPCs to torture and kill my son's PC.
And, to get this back on topic, I you use the book a lot, it will have some wear and tear. You can't avoid it, except by not using the book. But I think if you follow everyone's suggestions, you can still use your books but also keep them in really good shape. I think the spine a big issue. Once the binding goes, the book will fall apart, so don't crack the spine (the Crothian rule). Cover it to protect from spills (the morbiczer rule). And keep it out of high humidity & sunlight (the Ron rule). Don't let others borrow them (the Turanil rule). And keep them upright on the shelf when you're not using/reading them (the Barendd Nobeard rule).
Hopefully, some of our resident librarians (Eric Noah, Buttercup, Cthulhu's Librarian) can offer some tips.