Actually, I was thinking of posting a thread asking about the best non-fiction titles. I'll watch the responses here with interest.
I love "behind the scenes" info, and have the Captain's Log, DS9 Companion, TNG Companion, Voyages of Imagination, Making of Insurrection, Q's Guide to the Continuum, Everything I Need to Know I Learned from TNG, Delta Quadrant, Nitpickers Guide to TNG and possibly a few others that slipped my mind.
The last three are rather unimpressive, but I like the others. I'm trying to get the Compendium, but I don't know of any others that I should get aside from Harlan Ellison's rant about City on the Edge of Forever.
Oh, I also have the entire set of Star Trek Fact Files - that is, the one before the DVD edition, and I note a third version has just started...