This gives them a stamp of quality. Age - An old book isn't necessarily valuable. With books that have muliple editions, the first edition will usually be the most valuable. A second edition will contain different content to the first edition. Different printings of the same edition will usually be exact replicas, except for the print date.
Check dates to be sure of a first printing of a first edition.
The combination of a small print run and future fame of the author can make these books especially valuable. Collections - Single volumes of sets or incomplete sets are not usually worth much. They are better than regular editions. They are often signed by the author. Signed - Of course any book signed by the author is more valuable than one that isn't.