My two cents as a 20 year product manager with +10 enterprise applications under my belt (and having read several of these):

# "Don't make me think" is a seminal work on design thinking for online services. I've yet to come across a book with as much relevance and substance even though it was written for the dot com era.

# "Positioning" by Al Reis is a book I wish I read 15 years ago when I started my company... your product's strategic positioning will greatly inform and shape design decisions (typography, colors, tones, copy, etc)

# "Ogilvy on Advertising" - written by the legend himself, once you read this book, it will change the way you see all ads in any medium