Steven Pinker's "The Sense of Style" is a fantastic book about effective English writing. It helped codify some stuff I already did out of habit but never really knew why, and taught me plenty of new things too. Recommended for anyone whose job involves technical writing!

Alexander Watson's "Ring of Steel" is World War I seen from the perspective of the Central Powers. OK, it didn't really help me personally or professionally, so I'm not sure it counts. But it was definitely a huge perspective-changer, and so beautifully written too.