I've been a GURPS fan for a looooong time. Not quite the beginning, but certainly my entire adult life and then some. Many GURPS books have a reputation (deservedly earned, in my opinion) of being great resources even if you're not a GURPS player.
But a funny thing happened on the way to the future: the Internet. In particular, Wikipedia, which put literally billions of words of information (in English alone!) at your fingertips, for free. Getting lost in its web of widely accessible information is a cliché at this point. Does this mean that supplements like the GURPS Hot Spots series – which focuses on interesting locales throughout history – have been rendered moot? We don't think so!
Nowadays, the most important element of the gaming historical supplement is "gaming"; that is, presenting information that's useful to gamers. Sure, you might be able to find a description of rulers and years for (say) Constantinople. But quickly getting the most important details about a specific location's food, its sights and smells, its lived customs, possible plot points and mysterious happenings . . . that kind of information is much more scattered, and even harder to synthesize for the gaming table.
-- Steven Marsh