Lambda the Ultimate

There have been several threads recently about buying old computer science books, so I thought I'd make it into its own topic. What obscure books are out there that are worth searching for?

One that I've found interesting is "Lucid: The Dataflow Programming Language." There's also a followp, "Multidimensional Programming" (which haven't seen).

Occam books are also eye-opening.

And I have a copy of "Programming in Parlog" (a parallel Prolog variant) which I have yet to read.