As a developer turned co-CEO, developing an understanding of the differences between cashflow, revenue, profit, and overhead was critical.

My recommendations are a little different from most other folks here, in that they relate to understanding the business as a whole and how finance figures into it:

Norm Brodsky's "The Knack" http://theknack.info/

Jack Stack's "A Stake in the Outcome" https://www.amazon.com/Stake-Outcome-Building-Ownership-Long...

If you're in a startup environment, Brad Feld's "Venture Deals" https://www.amazon.com/Venture-Deals-Smarter-Lawyer-Capitali...