Turanil said:
If you are the only one to manipulate your books, and do it with care, they should be in good condition next 20 years.
My advice: don't let players who don't have their own books to touch yours. Get the SRD online;
Words to live by! People can promise to be careful, but that doesn't mean crap once they've ruined your book. Like I said in another thread, no one can promise that "nothing will happen" unless they are a god or Neo or something. And I don't think I've met "The One."
The DM of all people asked to borrow my PHB! I let him because the binding was shot anyway (not my fault, the first printing had a lot of shoddy spines

) and not two minutes later spilled some Sprite. Thankfully only a tiny bit got on the book, but it proves my point; no matter how responsible a person is, they can't make any promises. Accidents happen, but as long as it's me having the accidents with my stuff, then I just have to suck it up and accept it.
If YOU have the accident with MY stuff, I don't have to accept it; and then you have a problem.
Nobody touches my 3.5 boxed set, I don't care how much it was discounted!
Keep your books away from the floor. Tiny bugs like to eat books and if they're near the floor, it's that much easier for the crawlers to get to them. On that note, vaccuum often.

Low humidity, lots of air circulation... Those help keep a book in good condition.
No matter how careful you are when the book is in your hands, if you put a stack of them in your backpack loose, they will get damaged! Dog-eared corners, impacted corners, scuffed covers... Just from one trip to the DM's house. Amazon boxes are just the right size for D&D books. Imagine that! Line one with thin foam and you have a great way to transport your books. For Code Red Paranoiacs, put sheets of foam between the books so they won't get scuffed against each other .
Do not eat or drink near your books because, as was said, accidents happen.
Wash your hands. Pizza grease doesn't look as good as a Lockwood ilustration. Furthermore, just because you wiped "all" the grease off of your hands and onto your pants, that doesn't mean they're clean. Wash them!
This is why my old Magic cards and the like have pristine edges and a certain friend of mine's have greyish-black edges that look like the surface of a ten-year-old mouse pad.