Earliest _UNIX_ text: Kernighan & Pike, "The UNIX Programming Environment", Prentice-Hall 1984 is a must-have for anyone who wants to understand the UNIX philosophy of little tools working together. Today we'd use Perl instead of sed and awk, but the elegant style of these masters is well worth pondering.

I suppose my _oldest_ useful (and well-used) computing text would be my original John McCarthy's LISP 1.5 Programmer's Manual, circa 1962. I reread it (and parts of Patrick Winston's LISP (1 & 2)) just about every time I implement a new little language, because he and his students wrote the book on meta-programming.

UPDATE: b'damned, but I LOVE the Internet! LISP 1.5 Programmer's Manual is a scanned copy of the hand-typed version. Mine is only the first printed edition. :D