Many of the books I have came from others and some of the spines have been creased(paperbacks).  The books are not broken or coming away from the spine.  Are these acceptable?

Yes, spine creases are allowed.

As long as when you open the book you don't see the spine through the inside.  Like the pages are starting to seperate from he binding. But creases on the outside are acceptable.  I don't know how people read a book w/o the spine showing any evidence. I'm careful with my books when I read them and they always have some sort of crease on the spine when I'm done.  I don't flattem them out or anything either.

Thanks for the info....now my bookshelf can grow,grow,grow!!!