Appears to be a tie, between Programmer's Guide to the IBM System/360 (1969), and (Computer Readings Series) An Introduction To Computer Systems, apparently edited 1969, mostly a collection of reprited articles from the latter half of the 60's.

(neither of which I ever had cause to use in anger, of course ;)

ETA: An honorable mention: A FORTRAN Coloring Book, 1978 8-}