I'd like to recommend three of the Reeves-Stevens' titles: The Art Of Star Trek
, The Making of Deep Space Nine
, and Star Trek: Phase II, The Lost Series
. Oh, and the John Eaves co-authored Star Trek TNG Sketchbook: The Movies
is excellent as well.
Are any of them "important"? Probably not. The DS9
one is quite extensive though, so I'd say it's essential reading for any Niners out there.