Investigation of Rarity
If you are a regular collector of antique books, you can use different methods to determine scarcity or rarity of an antique book. For this you can search online, and you might find something written on the book you have been investigating. One commonly referred website is of Antiquarian Booksellers' Association of America (ABAA). They might have some material in written on the book's history and its topic. Also, you might be lucky to find a number of articles about the book, which should make your process of investigation a lot easier.
You can also determine if it is first edition of the book, although it is not an easier task. You can use guides such as "Pocket Guide to the Identification of First Editions," to determine the edition of the book. If you know of a different method, apply that to determine the edition of the book.
Inspect Binding of Book
Binding of book can also help determine value of an antique book. A damaged binding can for example decrease value of the book. Also, it is likely that the book itself is damaged in the damaged binding because with a damaged binding it is difficult for a book to sustain harsh weather conditions.
Authentication of Autographs
If a book is signed, you can authenticate autographs through different sources. For example, the people who buy and sell antique books can often verify autographs. You may also refer to ABBA website again or check your local directory to find a source to get the autographs verified.
Overall Condition of Book
If you are a regular collector of antiques books, you can determine their value by checking overall condition of books. A book might have a great historical significance but checking its condition is also necessary to grade it unless you know the subject it covers, and also know of some other clues that assure you that the book has a great value, no matter even if it is in a tattered cover.