The Cathedral & the Bazaar by Eric Raymond
The essay that started the OSS movement back in the late 90s. A thorough, detailed collection of essays that explain the concepts and strategic advantages behind open source software for both customers and producers.
My favorite quote was why companies should choose to use open source software when available:
“The brutal truth is this: when you key business processes are executed by opaque blocks of bits that you can’t even see inside (let alone modify) you have lost control of your business.”