If the main book isn't enough, Steve Jackson Games released three "Companions" in PDF format that expand on specific niches:
To expand on the main book, several specialized PDF "Companions" are available on platforms like DriveThruRPG : gurps low-tech pdf
The book rigorously defines the differences between technological stages: If the main book isn't enough, Steve Jackson
is highly recommended over the print version for a few reasons: Searchability If the main book isn't enough
Here you'll find all collections you've created before.