A book isn't truly collectible unless it is a first printing of a first edition. Most titles will have only one edition while being in print, yet some have as many as 42 or more printings, with the first printing being the only one of value.
The back of the title page contains a string of numbers referred to as the number line. The number line identifies the print run of the book. Because the publishing industry has not standardized this practice, there is much confusion over how to interpret this number line.
Each publisher has its own form of identification: 1 3 5 7 9 10 8 6 4 2 is an example of a number line from Anness Publishing. The 1 indicates that this is a first printing. In later printings, the lowest number is removed as the title reprints, so this same number line in a third print run would look like this: 3 5 7 9 10 8 6 4.
Some publishers reverse the order. For example, 10 9 8 7 6 5 4 3 2 1, where the 1 at the end of the line indicates a first printing. This same title in a second printing would be 10 9 8 7 6 5 4 3 2.
Some paperbacks are more valuable than hardcovers. Sometimes as a series becomes popular, the publisher will reprint the earlier books in hardcover editions. In this case, the paperback first edition is the one to collect.
There are many guides available to help you identify first editions. Bill McBride is the author of two guides small enough to carry with you while you are out book hunting: "Pocket Guide to the Identification of First Editions" and "Points of Issue: A Compendium of Points of Issue."