http://e23.sjgames.com/item.html?id=SJG37-1664 This book is, to some extent, fallout from GURPS Low Tech. While writing Low Tech, we ruthlessly ruled out all kinds of speculative, discredited, and romantic low tech inventions. This book contains all of them and more.
The Edge of Reality continues my trend of books which do just what they say on the tin. Everything in it is from the fringes of what's real: imagined but impractical inventions (think Leonardo da Vinci's notebooks), misinterpretations by earlier scholars, prototypes which never caught on, and so on. In a way, it defines GURPS Low Tech as negative space.
It's also, in its weird science-ness, probably as close as I'll ever get to covering the same ground as Ken Hite.