GURPS Psionics is, in my opinion, one of the best GURPS books written. First, it's a good, solid set of rules for psi, and it's easily customizable for any campaign. Second, it has a LARGE section on how to fit psi into a new or existing world, including several very good suggestions on how to explain the origin of psi. Third, it has a very well-written "sample" world, which can easily be used just the way it is. And fourth, it has the bibilography at the back almost chararistic of GURPS sourcebooks, containing nearly every book, comic book, movie, and TV show ever written on psionics. I consider this an essential book for any GURPS campaign on psi - while the section in the basic set (third edition) is fairly good, it is incomplete at best when used for a settign ABOUT psionics, although for a campaign with psi tacked on, just for a little spice, the basic set could suffice.