The Star Trek Concordance is a reference book written by Bjo Trimble. The concept for this book was based on a privately-printed fandom publication established by Dorothy Jones Heydt in 1968.
It was first professionally published by Ballantine Books in 1976 as an unofficial companion to TOS and TAS. It was later endorsed by Paramount Pictures.
The Citadel edition, published in 1995 was updated to include summaries of the first seven movies and key Star Trek: The Next Generation and Star Trek: Deep Space Nine episodes featuring TOS characters. However, the revised book was published too soon to contain any details from the DS9 episodes "The Sword of Kahless" and "Trials and Tribble-ations", the Star Trek: Voyager episode "Flashback", or Star Trek: First Contact.
The Concordance cross-referenced episodes, characters, alien species, etc. from both series. Trimble gave animated material equal weight to TOS in this book, and also included fan artwork and illustrations, indicative of its origins as a fandom publication.
Unfortunately there are mistakes throughout the book, especially in the cast lists. These lists were copied to the later editions of the Star Trek Encyclopedia with the errors remaining. This has also led to fan sites listing incorrect information. There is no word on whether of not these errors were corrected in the updated release.