The easiest series to pick up that have been properly collected (at the time this was written) are:
Firestorm the Nuclear Man TP (There is no Comic Vine article on this series) This book collects the 5 issue mini series that first introduced Firestorm, as well as some of his back up stories from the Flash. It is a great into to the character, and offers some very good classic Firestorm reading material.
For a more modern Firestorm story, pick up The Fury of Firestorm: The Nuclear Man - Takeover written and illustrated by Dan Jurgens, vol 3 of Firestorm's New 52 series gets the character back on track after a rocky start in the New 52. It's a good book that's a lot of fun and makes for a good read, written by an author who understands the character.