If Donald Knuth, The Art of Computer Programming counts as a computer book, then that's the oldest one I have, for the translation of volumes 1–3 is based on the second edition from 1981.

Otherwise, it's one of the thick volume IBM Personal Computer XT Technical Reference (1983), or the digital copy of the blue book Adele Goldberg, David Robson: Smalltalk-80: the language and its implementation (1983) which you can download (scanned) from http://stephane.ducasse.free.fr/FreeBooks/BlueBook/Bluebook.pdf.