“Sprint offers a transformative formula for testing ideas that works whether you’re at a startup or a large organization. Within five days, you’ll move from idea to prototype to decision, saving you and your team countless hours and countless dollars. A must read for entrepreneurs of all stripes.” —Eric Ries, author of The Lean Startup

We did design sprints at Weeronline.nl and Zoover.nl, and I found this one browsing Amazon.
Nice to read, not that hard to understand.
Hard to do though: setting up the right challenge with the right people is hard for example.
Half way, will update later