The cover had a blueprint design, as I recall.
Does anyone know its full name/author? I would try amazon.com but there was almost 20,000 entried under "Star Trek".
It was pretty damn cool. It had artwork of the ships, and a fun read since it was unofficial, and thus different than the "approved" ST timeline. The entries were more like newspaper clippings in their wording than as an encyclopedia.
BTW I also own a TOS medical guide which has the anatomy of the Tribble.