The book which gave you knowledge that propelled your life in a meaningful way.
SICP of course, as for most others also
I was going to say One, Two, Three, Infinity by George Gamow, but that's not a textbook, so what stands out in my mind are the Feynmann Lectures.
When I started out in computing, think 8080 and 6502, there were basically no books. On the optics end, which eventually gave way to computing, probably Modern Optical Engineering, by Warren Smith.