I tend to look at my books as my collection. Collection in the same way someone would collect action figures, and keep them in a box for value. My books, my collection, has a very high value to me. In this way, I prefer to keep them as clean as possible - I want to have them forever. I'll never sell off or put a monetary value to them, and to me they'll all be priceless.
Having said that, I like looking at a section of worn books (my Anne Rice paperback novels) and see the multitude of cracks in the spine and just know I had read them over and over and kept them in arms reach throughout a time. There might possibly be a chocolate or a candy stain, but that's alright, because it's MY stain. It is MY book and it's okay for ME to mess up MY book.
I have a book, Bitten - By Kelley Armstrong. It is tragic how battered this book is. I would be upset, however!; I've lent this book to about 5 different girls and got them all hooked on a fabbbulous series. So, even though it pains me a little to see an unfamiliar stain, and that someone had dog-eared the book to keep place, it shows how enjoyed the story was. Would I buy another Bitten to look good on my shelf? No. Had I lent the latest of her books out and it came back battered - yes, yes I would have.
I like to go 'treasure hunting' (thrift store hopping) to find hardcover editions of books/series I discovered too late. Harry Potter, for example, I had discovered when my grandmother bought my brother the softcover of Philosphers Stone. As I caught up to new released, I would buy the hardcover, and I've been digging for perfect looking hardcovers to complete the series. Likewise, I've been searching for AGES for a hardcover of City of Bones/City of Ashes.
I love to collect books of stories I've read. There are very few, in fact, I think only 3 books total that are on my shelf that I haven't read - yet. This gives me the missing motivation to read them so I can confidently say EVERY book on my shelf has been read and enjoyed by me.