The original GURPS Illuminati was rough, but a fun book of inspiration for wild conspiratorial world-girdling secret society weirdness.
When Kenneth Hite started working for SJ Games, he produced a great collection of well-researched and well-written alternate-history weirdness:
- I find GURPS Cabal to be a masterpiece. If you want World of Darkness but less emo goth and more crossed with Kabalah and Illuminati and multi-planar divine power.
- There's great value in each of GURPS Alternate Earths and GURPS Alternate Earths 2. A few pages fleshes out each entire rich alternate world history, each one well enough to build an entire campaign.
- He ran a regular column, Suppressed Transmission, in the SJ Games magazine. There is a Suppressed Transmission anthology that is well worth dipping into.