Star Trek Chronology: The History of the Future
by Michael and Denise Okuda, published by Pocket Books, 1993. Revised in 1996.
This book is a fascinating look into the various histories surrounding the various Star Trek series and movies up through The Next Generation. Growing up as a Trek fan, I was always very impressed with how consistent the timelines were, and how all of the "historical" events in the various plot lines ended up making sense and not walking all over each other. This book really highlights that: The authors go through all of the episodes of Star Trek and Star Trek: The Next Generation and pull out all of the timeline references, putting them into a consistent chronology that follows the Trek history from "Modern Day" through to (In my 1993 copy) 2368, when the fifth season of TNG was set.
This book goes into remarkable detail. It highlights historic events, character birth dates, other significant character dates (graduation, ship assignments, etc), and even manages to connect various alternate timelines, such as the Sela storyline in TNG from Yesterday's Enterprise. It even includes a very cool timeline chart that shows the significant character events in relation to each other. If you've ever wanted to know that The Wrath of Khan took place in 2285, that Natasha Yar escaped from the Turkana IV colony in 2352, and that Qpid, the Robin Hood episode of TNG took place on Stardate 44741.9 in 2367, this is a great book.
Unfortunately there hasn't been a followup book that I can find for the rest of the Trek storylines, but for the neurotic Trek fan in all of us, I can't recommend any book more highly than The History of the Future. I find it "useful" as an episode reference, as well as a source of insight into character backgrounds and motivations.