"Basic Set Rule Changes from the Second Edition to the Third Edition"
This 80-page book was maybe the most unusual book in the whole GURPS line. It was also one of the most useful.
From the forward:
"... when the Third Edition of the GURPS Basic Set came out, it was a massive revision. There were far too many changes for a simple revision sheet. ...
"Therefore, this Update. It compiles, as best we were able to judge, all the significant rules changes, additions and deletions between the Second Edition and the Third."
It also contained material to update GURPS Japan, Space, Humanx and Horror to Third edition as well.
Finally, it contained all the changes that were made in GURPS Magic that superseded the first printing of GURPS Third Edition so that Update even served as an upgrade to the first printing of GURPS Third Edition!
One thing lacking from it was the usually copious artist credits. There's art, but none of it's credited, though all of it appeared in previous GURPS publications.