...that you never actually play? Whether it's a 'just collecting', 'haven't played in ages', 'can't find a group', whatever.

Me, it's Modiphius' Star Trek Adventures. After I posted the last thread about Trek games, I was able to get a good deal on most of the books; I think the only things I'm missing are the Player's/GM's Guides, the Klingon core book, and the Disco sourcebook; and I'm not too broken up about missing those. Even scored the tricorder collector's set!

Got to thinking about it because Modiphius announced STA 2E recently, supposedly back-compatible with all the 1E books, but they've already previewed a couple pretty significant mechanical changes and I'm wondering how the conversion is meant to work. Surely they'll get to that in later previews.

...it's probably Shadowrun. Had a good run playing back in the "good ol' days," and have books out the wazoo as a result. Group fell apart and stopped playing, though, and other than a few online romps in the late 90's early 00's, haven't played since. Gamma World and Gurps come in at the top 3 as well.

A new acquisition and exactly one book, but I'm pretty sure I'll never get around to playing...Pendragon 6E is finally out! Arthurian mythology is what got me into fantasy and gaming in the first place as a kid, and I've never been able to find the previous editions' books for a reasonable price. I've been looking forward to this one even just as reading material!

I have a lot of sourcebooks for multiple versions of D&D, from 3.5 back to OD&D.  I only really play 2e, and truth be told, I really just use my PDFs.

Decided to drop into the Inn and see if anything was happening. Saw this post, so . . . 

D&D easily.  I'm sitting here staring at a stack of 12 books across the room.  And there are more 5e books around.  Then there are the 3.5 books that I got when I first got back into gaming after a decade off.  Then there are 10 or so original AD&D books from the 80s.  Then there are the Basic AD&D books from the orginal box sets.  Do novels set in the world count?  I don't actually have a lot of those, but I do have the first 6 Dragonlance or so up in the attic.  I assume you only mean books, so adventure manuals and magazines (Dragon Magazine, anyone?) don't count.  Even i don't have enough dice to count up all the money I have spent on books for this game.

And it has been worth every penny.