Eliyahu M. Goldratt
Reading The Goal was a fantastic journey into thinking about lean without getting bogged down in anything technical. The Goal is a great novel and a long way from a dull technical book, it really gets you thinking about the core ideas behind Kanban. Following with a more specific book on the Theory of Constraints defines the lessons learnt in The Goal and adds ideas on how to manage your own constraints in the system.
David J. Anderson
David J. Anderson is one of those leading a march towards Kanban and Lean adoption in software development and his Kanban book is a great guide to applying Kanban to your existing process and identifying opportunities to improve.
Alan Shalloway et al.
The Lean-Agile software development book includes the experiences of the authors extending agile practices into the wider organisation by adding lean techniques to the mix.
Douglas Adams had a fascinating way of looking at the world and always came up with brilliant ways of conveying that vision. The way that Douglas would talk through Dirk Gently about the interconnectedness of all things really did grow my ability to engage in system thinking. Reading the two Dirk Gently books: Dirk Gently's Holistic detective Agency and The Long dark tea time of the soul will help get you in a lean thinking way.
There are of course many other good books on Lean, but these are the ones that have suited me best so far (although there is still a lot to read).