Author's autographs in a book may be considered in various categories, including signed limited editions, signed trade editions, and association copies. We've discussed limited editions elsewhere on the site.
Signed trade editions are copies of the regular trade first edition signed by the author, with or without an inscription. These signed books will usually sell for at least twice as much as an unsigned copy, but the real determinant of price will be the value of the author's signature. Some authors are very generous in signing their books; as a result, their signatures may be worth only $10 or $15, representing the price difference between a signed and an unsigned copy of a first edition, or the price of a signed later printing. On the other hand, some authors very rarely sign a book and their signatures alone may be worth $50 or more; again, this would establish a price. Further, some authors are very free with their signatures but very rarely inscribe copies of their books, and therefore inscribed copies, even if the recipient is unknown, will command a premium.
Value of Signed vs. Inscribed Books
Association copies are books that include a signed inscription from the author to another famous personality or someone important within the framework of the particular author's life and work. These will be valued more highly than the normal signed first edition, depending on the importance of the recipient involved.