My pick is:

Rework by Jason Fried & David Heinemeier Hansson of 37Signals. After 10 years, it's still an amazing book filled with incredible nuggets for just getting stuff done. I revisited it recently what with all the hype around HEY.

Some other books I thought about picking include:

Sprint by Jake Knapp, John Zeratsky & Braden Kowitz of Google Ventures looks at how to test ideas fast.

Rocket Surgery Made Easy by Steve Krug gives practical instructions for conducting in-person usability tests.

This is marketing by Seth Godin stopped me seeing marketing as something other people do.