Ash Maurya presents a step-by-step guide to implement the Lean (Startup) methodology. It's far more down to earth and practical than the most famous Lean Startup by Eric Ries. The latter book introduced thea language to the already existing world of product design-by-experimentation. But it was fairly general and in many ways difficult to apply.
Enter "Running Lean" by Ash Maurya, which precisely describes a process (a procedure even). It tells you what should you do, when to do it, what are the stages in the Lean process and shows some caveats.
Perhaps it's even too detailed. Using a very specific examples from a website project it tries to generalize them which may be not applicable in many cases (as the Lean process itself).
Being a step-by-step, practical manual this book is also difficult to read. I couldn't consume it in one seating as it's, well... a manual.
If you're just looking to learn about Lean Startup movement and method for the sake of knowing, Lean Startup by Eric Ries is a far better read. The Lean Startup: How Today's Entrepreneurs Use Continuous Innovation to Create Radically Successful Businesses
All in all, a must-have (as opposed to must-read) for people planning to apply Lean Startup process in their projects. What's important is that it's not overly detailed, concise and practical.