"Magic System and Game World"
Maybe the most important GURPS supplement ever. 100 pages.
When the first GURPS Basic set was released, despite its high-sounding goals of being able to run any campaign in any time, it was mostly geared toward traditional fantasy.
While the GURPS combat rules had been vetted in Man to Man and the Basic set had a solid attribute, skill and advantage/disadvantage system, the Magic system got all of a paragraph -- saying it would be covered in GURPS Fantasy.
Fantasy was released about 4 months after the Basic Set. In it was the GURPS magic rules consisting of over 300 spells organized by colleges with logical prerequisites (you need to know how to magically light a fire before you can cast a fireball, you need to know how to restore fatigue before restoring damage, etc.). Rules for alchemy and for enchanting items were included as well. The magic system was integrated into the GURPS combat system.
The second half of the book contained the world of Yrth, a fantasy campaign setting. It was this part that would continue on in later editions of GURPS Fantasy -- the Magic rules being split off into GURPS Magic.