Books signed by the Author
Another thing which makes a book more valuable is if it is signed by the author. If a book is both a first edition, first printing, signed by the author it is highly collectible if it is in Verfy Good shape.
Be aware of forgeries! Some unscrupulous sellers will forge a signature in order to get a higher price. To avoid this, insist on seeing the autograph prior to purchase, and compare this to the "real" autograph of the author. There are several sites which help you make this determination. (use Google to search for authenticate signature or use the name of the author such as "John Steinbeck signature"). John Steinbeck seems to be widely forged to check carefully!