For the 4th edition, in that order:
- GURPS Lite
- GURPS Basic Set
- How to b e a GURPS GM?
- GURPS Martial Arts
- GURPS High-Tech
- GURPS Horror
For the 3rd Edition:
- The two Compendia, even if they are not useful since the 4th edition.
- GURPS Goblins
- And many others.
Yes,
GURPS Goblins is a musth have! There is a lot of humor inside, of course, but also a lot of little things that can be useful in every campaigns, like the brilliant rules to determine what is the weather like with a dice roll.