4 Steps to the Ephiphany - http://www.amazon.com/gp/product/0976470705
Entrepreneurs Guide to Customer Development - http://www.amazon.com/gp/product/0982743602
Lean Analytics - http://leananalyticsbook.com
The Lean Entrepreneur - http://leanentrepreneur.co
Lean UX - http://www.jeffgothelf.com/blog/lean-ux/
The Lean Product Book - http://www.amazon.com/gp/product/1449319955