I have younger siblings (when I started the game, my sister was 8.) So some of my older books are just about deceased (Plus, having to lug them around in a backpack every week (my fanatical co GM insists on me bringing every book.) doesn't help their condition- though now I'm making excuses...). My Hero Builder's Guide is missing a cover, and the pages are bent and ripped. But it's a soft-cover so that's expected.
As far as my Hardcover books go, it's a pretty cut-throat competition among some. My Monster manual, which on the outside appears to be in the best condition of the three core rulebooks, actually has one page that's been ripped and the entire contents of the book have spilled out and have had to be taped back in.
My Player's Handbook and DMG are both in fairly bad shape. The PHB says "Player's Handboo-" on the spine, and the DMG's spine is nekked. My PHB has my sister's name written along the pages opposite the spine (if that makes sense) in marker. Of course, like every other older brother I simply made it worse by writing "sucks" after it, but still...
Both books are also suffering from being pretty dinged up along the edges as well, and the PHB has had a page torn, and I can't find the rest of it. But I didn't really NEED Hallucinatory Terrain....
As far as non corebooks go, just about everything is somewhat dinged at the edges unfortunately, but none can match the disrepair of the three, the proud, the "oh who cares I'm replacing them with 3.5 sooner or later anyway"!