The way things can be collected today would indeed seem like fantasy back then. We just couldn't envision those things being so accessible and (relatively) inexpensive.
Things I still have from those days:
- The Making Of Star Trek
- The World Of Star Trek
- Star Trek Compendium
- Star Trek Concordance
- Booklet Of General Plans
- Star Fleet Technical Manual
- Star Trek (by James Blish, first adaptations)
- Star Trek Log series (reprints)
- Spock Must Die (by James Blish)
The TAS Star Trek Log adaptations are reprints but I did once have the original first printings. I also have the reprinted collections of the Gold Key Comics I collected back then.