I'm a big book lover, and would not at all be offended by use books. One thing to keep in mind though, is how old the book is and if the recipient has allergies. I am allergic to books whose pages have gotten musty--my eyes water, my nose runs, and I sneeze. It's a huge pain in the butt when I'm shopping for books at garage sales--I'll find one that's awesome but if the pages are yellowed and sort of musty I know I'll never be able to read it for more than 10 minutes.
So yes to used, no to being used to the point of producing allergens-lol.