A new GURPS book came out this week - GURPS Power-Ups 9: Alternate Abilities.
If I had to sum the book up in a sentence or two, it's a discussion of changing the price - or the nature - of existing attributes and secondary characteristics. It's also a discussion of how to add additional abilities to the canonical attributes and secondary characteristics.
Basically if you ever wanted to re-cost GURPS attributes or add new ones, this book will help you do so with a solid idea of the pricing decisions involved. It's a very wonky book for system-hackers.
I received my copy as a complimentary copy for providing some peer review feedback.