If I picked up the sourcebook GURPS Thaumatology, would I need GURPS Magic? There seem to be a lot of negative reviews of 4th-edition GURPS Magic and nothing but rave reviews of Thaumatology.
According to "Introduction: Current Cross-References" (GURPS Thaumatology, p. 5), the book is designed to stand alone and doesn't need GURPS Magic. The book presents alternatives to the spell system provided in the Basic Set, as well as some modifications of the default spell system.
If you are using the spell-magic rules from the Basic Set, then GURPS Magic will be relevant (though not needed), but you could easily turf the generic spell system entirely and implement magic in your game world with one of the Thaumatology magic system alternatives.
Thaumatology is a magic-system construction guide. You're very unlikely to be using the whole of the book in one campaign, any more than you'd be using all of Fantasy or Space. Some of its advice is relevant to games with Magic-style spellcasting, other bits aren't. So the answer is "it depends on the sort of magic you want in your game".
If you want something like traditional dungeon-bash magic, lots of showy stuff achieved relatively quickly, you'll want Magic; if you want to tweak it in detail, you'll also want Thaumatology. (The downsides of Magic are mostly that the text wasn't sufficiently proofed before publication, so much of it is the same as in the 3e magic books with some of the old bugs still present, and the artwork is a bit naff.)
If you want subtle magic that takes a while (as in the old Voodoo and Spirits books), or if you want potentially world-altering magic like the sort of thing you'd meet in a Mage campaign, you won't need Magic too.