Already been mentioned: The Making Of Star Trek
and Inside Star Trek
as well as The Star Trek Sketchbook
are the best. The Star Trek Compendium
is also nice to have.
After that The World Of Star Trek
and some of the "making of" the films books are okay for maybe picking up tidbits.
Although they are not behind-the-scenes books there are works that I think are important in other respects. Franz Joseph's Starfleet Technical Manual
and Booklet Of General Plans
are important works in that they really crytallized and fuelled fan fascination with treknology (even if a lot of the materiel later proved to be questionable in terms of authenticity and consistency with what was seen onscreen). Also equally good was the Star Fleet Medical Reference Manual
and the Star Trek Concordance.