[kqr]

At first I was going to say "there are too many good ones and I can't pick out a favourite." But then I looked at the list and there is one clear favourite among many very good ones: Performance Modeling and Design of Computer Systems.

Queueing theory is probably one of the most useful subjects of all time, for just about any human. This book teaches it in a way that made it extremely easy for me to grok.