Most Useful LIS Textbook

From the perspective of someone who actually got my MLIS 13 years ago, I can say that the textbooks that I bought back then that I actually still use today are the Anglo-American Cataloguing Rules, The C Programming Language by Brian W. Kernighan and Dennis M. Ritchie, and The SGML Handbook by Charles F. Goldfarb.

If I was going to recommmend LIS book over all others, it would probably be Second-Hand Knowledge: An Inquiry into Cognitive Authority by Patrick Wilson.

The only book I haven't sold, gave, or burnt is Fred Schlipf's amalgamation of library buildings information. It's just an oversized handout (by a couple hundred pages), and I love it.