I no longer refer to it daily, but I still keep my ITT Advanced BASIC manual (1985 printing) where I can reach it -- not because I ever program in BASIC any more, but because the appendices have other, semi-related, useful information. It's all info that's also readily available elsewhere, but in that book I know right where to find it quickly, because I've been referring to it there for years.

Apart from that, the oldest computer-related book that I use with any frequency, in any capacity, is my K&R2 (1988), which is exactly the right size and shape to support a mouse pad, so I use it for that any time I need to use a mouse elsewhere than on a desk. At one time I tried to use it in a fashion more usual for a book, but I found it rather lacking in that regard, simultaneously verbose (in explaining things every programmer already knows, regardless of linguistic background) and yet FAR too terse (in explaining the language it's supposed to be documenting). I think the lack of a decent book is one of the main reasons I never learned to make myself like or use C. (Another is that the language is inherently rather low-level, and I tend to want to solve higher-level problems. However, that can't be the whole deal, because, with a good professor and a decent book, I had no trouble picking up and enjoying 8086 assembly language, which is also fairly low-level.)

I've also got two volumes of Knuth on my shelf (the most recent date on the first volume is 1973), but I can't speak for its usefulness, because I never got around to reading it. I wanted to learn C first, _then_ read Knuth, and the first project never got done, so I never got around to reading Knuth. Similarly, the compilers book with the dragon on the cover is sitting around here someplace; I picked it up (used) around the same time that I picked up Knuth, expecting to read it after I learned C, which never happened.

The most useful computer-related book I've got is the (teal) camel, but that's not very old.