22. Best secondhand RPG purchase.
Hm... Could go for the splendid revised edition of West End's Star Wars game, the so-called "super mondo" edition, but I don't get to play it often. Or maybe The Traveller Book, the very comprehensive hardback compilation of GDW's Traveller sci-fi rules, one of those rare games you could play forever with only the one purchase.
No, I think this has to be a supplement: Strike Force by the late and greatly missed Aaron Allston, a Champions sourcebook and campaign guide. Picked it up a few years ago quite cheaply and in great nick, which is increasingly difficult to do as this book is much sought-after and very highly regarded. It's really a published version of the notes for Allston's legendary Champions campaigns, but expanded to be so much more than that, a truly excellent volume of advice for GMs even if you don't play Champions, or even superhero games at all; after reading this you might find yourself itching to slip into a pair of tights and create a secret identity anyway, but hey, it is the weekend