The value of an old book can be approximated through print sources, such as "Bookman’s Price Index" and "Handbook for Booklovers." In order to determine the exact value of a book, it's necessary to have it appraised by a professional who has expertise in antique books.
Not all old books are considered valuable. Factors that determine whether a book is valuable include the importance of the text, the book's condition and whether it's sought after by libraries and collectors. In general, American books printed before 1801, English books printed before 1641 and all books printed before 1501 are likely to be valuable.