Usually, when a non-fiction author writes a followup to a classic that covers the same ground... the result isn't so hot.
But David Allen's "Ready For Anything" is a really pleasant surprise.
Definitely read "Getting Things Done" first, if you haven't yet. But then Ready For Anything is surprisingly good. They're little one and two pages essays that clearly explain some principle about organization, and have a few good quotes mixed in.
"It's possible to own too much. A man with one watch knows what time it is; a man with two watches is never quite sure." -- Lee Segall
"If not controlled, work will flow to the competent man until he submerges." -- Charles Boyle
Arguably, the quotes alone make the book worth it -- Allen obviously collected them for years, and put together the best and most on-point about prioritizing, organizing, keeping clutter down, keeping your mind clear, and doing great work. Combined with a basic principle and a few questions to think over and some actions steps each chapter, and I'm really pleased with it.
Getting Things Done is still the much better starting point, but you've probably already it. If you have, Ready For Anything is a really solid short read.