Maybe you were short of money and you sold it. Maybe you were short of space and gave it away. Maybe you lost it in a move, or the dog ate it, or it was destroyed in an overflowing toilet incident.
Whatever the reason, you no long are in possession of this beloved RPG book. What is it?
I have stupidly gotten rid of many books over the years, and I almost always regret having sold or lost all of them at some point or another (even if it is just because I have an itch to read it for nostalgia sake). But the one that really bugs me that I wish I had never parted with is the Star*Drive setting for Alternity. It is one of my favorite space opera settings and the book was really fun to read: thick and dense and good for just diving deep into. There are other books -- I gave away all my author copies of GWd20 PHB for example -- but I actually miss that Star*Drive book at least once a year.