I strongly recommend cindy alvarez' "lean customer development" which is spot on: http://www.amazon.de/Lean-Customer-Development-Building-Cust... ... In fact the lessons learned from this book are invaluable I'd say. If you want to be serious about product design for endusers, read this.

When doing the UI/X later, this book helps a lot: http://www.amazon.de/Dont-Make-Think-Revisited-Usability-ebo...