I'm not sure how many I have, but somewhere in the region of 20-25, I think? Some of them are for specific artists - Alphone Mucha, Shaun Tan, etc. - others are more general for art movements - Art Deco, Impressionism, etc. - and yet others are for certain videogames and/or movies - Okami, Final Fantasy IX, Ghibli-movies, and so on.
I don't know how old the oldest one is, but the one I bought first was the Final Fantasy IX-artbook. This one:
... which might be out of print now, or at least hard to get ahold of? It's pretty amazing, anyway. It contains everything from character sheets to environment sketches to object designs, AND some nice Amano-painting right at the back.
Art books are nice, especially if they contain a lot of development-sketches and stuff - it's nice to peek at the process of other people - but most of my shelf-space is taken up by other books. Novels, poetry, non-fiction books, field guides to birds, etc. I'm a bibliophile, and I am perpetually running out of bookshelf-space.