In a readable and accessible style, John Shook provides an excellent introduction to the "A3" plan for problem solving. It is the most accessible and understandable beginners level book I have seen on the subject.
Pascal Dennis' "Getting the Right Things Done" also covers the A3 process - though for strategy deployment. Personally I find John Shook's "Managing to Learn" more readable and a better introduction to the A3, though they do cover different ground and the one doesn't replace the other.
If you want to know more about the A3 planning and problem solving process - that ties everything in lean together - this is a great place to start.
John Shook maps the elements of A3 but more importantly the standard thought processes employed in effective A3 problem solving nicely. The book focuses on the thinking and the "why" rather than being prescriptive in the activities and the "how". This approach prepares the reader well for the application of A3 thinking and for the long journey towards personal mastery.
An easy read that gives a great incite into using A3 as a leadership coaching device both from the perspective of the coach and the coached. The book also forms a wonderfully easy to use reference text to keep you on the path.