Something that's antiquarian is antique, and usually collectible. Antiquarian books are used books that have some value because they're rare.
If you go into the antiquarian business, you'll probably buy and sell old things — antique furniture, paintings, or coins, for example. An antiquarian book store has unusual and valuable editions of books for sale. An antiquarian is someone who specializes in, studies, or collects antiques. The word comes from the Latin antiquarius, "pertaining to antiquity."