Comment The Advanced User Guide (Score 1) 624
The first book that got me thinking deeply about computing, not as a collection of hacks and tricks, but as something to be done properly. It's not a programming manual as such, but it's chock-full of advice-by-example, illustrating some of the really good bits of design in the Beeb, how the various subsystems interacted with each other, how to make best use of extremely limited memory without sacrificing design -- and the value of writing something well, which could allow it to be used in ways you never thought of.
I've read many great books since then, of course -- K&R, Knuth, Sedgewick, Programming Pearls, Effective Java, Programming in Scala, and more -- but the Advanced User Guide has probably had the most influence on me and how I think about computers. It also has the most annotations and other scribblings all over it!