I may have one of the more obscure collection interests, but ever since I discovered the wonders of P.G. Wodehouse I've been a collector of his books, the vast majority of which are now out of print, and the original editions of which are often rare and very hard to find. Not that my collection is worth much monetarily -- except to me. There are quite a few ordinary paperbacks (albeit in excellent condition) which are worthless to any serious collector except just for reading, but also some original editions going back to the 20's, 30's and 40's, some with the original dust jackets. The original editions are the actual collector's items with real value.
Like a lot of things that people collect, it's enticing because the original items themselves are aesthetic and historical.
Even Sothebys got into the act when the James H. Heineman collection -- the most famous in the world -- went up for auction in New York in 2001. Not that I could afford them, but I could dream!
"Everyone is entitled to their own opinion, but not their own facts."
Daniel Patrick Moynihan