After reading the book Lean UX, I had the same impression I had when the term Design Thinking was on everyone’s lips: Although there is good value in the approach as such, most of it is not really new. It is all about iterative design, personas, asking the right question and making sure, that you solve the right problem. Going a step further, Lean UX is focussing the integration of UX design into agile development cycles and on fostering team thinking.
The foundations of Lean UX are Design Thinking, agile development and the lean startup method.
Anyway, having all these principles written down in a small manifest helps you to get the right things done – fast, iteratively and as a team. Read my short Lean UX overview (PDF) about the progress or pin it to the wall.
The Book: Lean UX by Jeff Gothelf with Josh Seiden