2e Chronomancer.
I know of roughly a dozen people (myself included) who
own a copy but zero who've ever actually used any part of it.
Interesting/odd read, but really needs stuff be written to encorperate it.
I fell in love with this book, and it ignited something in me about the concepts of time etc. that have held over for +20 years! This and the Complete Book of Necromancers! But your right of course, I had a GM tease me that he'd let me use the Chronomancer stuff, but that fell through. But every edition I look for a way to make one nonetheless.