I have a pretty good assortment of the early books, including The World of Star Trek autographed by David Gerrold's at WorldCon in '06.
The Making of Star Trek
The Trouble With Tribbles: The Complete Story . . .
Starfleet Technical Manual
Star Trek Lives!
Star Trek Concordance
Star Trek Action Toy Book
Star Trek Puzzle Manual
Letters to Star Trek
Meaning in Star Trek
Trek or Treat
Star Trek Quiz Book
The Star Trek Puzzle Manual
Starfleet Medical Reference Manual
The Making of the Trek Conventions
Star Trek Speaks
On The Good Ship Enterprise, My 15 Years With Star Trek
and the first novel, Mission to Horatius (not the reprint)
One other item I'm pleased to have, though it's not too old (from 1991), I think it's fairly rare. It has a rather long winded title; "Star Trek An Annotated Guide to Resources on the Development, the Phenomenon, the People, the Television Series, the Films, the Novels and the Recordings", by Susan R. Gibberman.