Kent M. Pitman has restored and made available on the net The Revised Maclisp Manual (The Pitmanual), that gives great insights into MACLISP, which had a great influence, both directly, and indirectly via Zetalisp (the Lisp Machine Lisp), on the evolution of Common Lisp.

For those interested in such things, the Lisp Machine Manual (Chinual) is also a rich source of information on the MIT Lisp Machine Lisp, before it diverged into different dialects in the Symbolics and LMI commercial Lisp machines.

(Via Geoff Wozniak.)