I have about 10 shelves-worth of mostly horizontally shelved Trek books, and not nearly as many as I want.
For a long time, I wasn't going to bother with TOS novels. I finally gave in to the inevitability of my OCD tendencies. So now I pick them up when I can find first printings in decent condition.
I have all the TNG numbered novels, though I'd like to replace a few that have broken spines. That's a low priority, though.
Three of my shelves are double-stacked with Trek books in back. I'm really running out of shelves.
DS9 and Voyager numbered novels are some of the hardest to find in used bookstores, especially in good condition. What few are out there were apparently read roughly.
I'm constantly trying to decide what order to shelve the books. For the last couple years I had them shelved by print date, but today I went back to series, then print date, and all the non-show series, series crossovers, and just plain oddballs at the end. Don't know how long that system will last.
I can't tell you how much I love that almost all the trade paperbacks are the same size. And I can't tell you how ANNOYING I find it that the hardback sizes vary like crazy. However, the worst is that ONE trade that is taller AND wider than all the rest. WHYYYYY?!?!? Why do you people hate me so hard?!?!?
I know most of this shelf is Star Wars and Earth: Final Conflict, but there are a couple stray Treks mixed in there.