The first book discusses why lean is a good idea and how it applies to software development. It is meant for skeptics who need to be convinced. The second book assumes that the idea of lean/agile software development is accepted - so the reader is saying to themselves "Okay, now what do I do about it?" The second book can be read alone by someone who has bought-into agile development. If not, start with the first one. There is almost no overlap between the books - we worked hard not to repeat ideas that were already presented in the first book, at least, not in the same way.
Author of Lean Software Development, Implementing Lean Software Development, and Leading Lean Software Development