I read Six Thinking Hats directly after reading Lateral Thinking. Both are really excellent reads, but I discovered that I knew quite a bit about lateral thinking already having been exposed to that mode of thought through many people I know socially and have worked with over the years, in some respects it is a natural way for me to approach problems already. Whilst I highly recommend Six Thinking Hats, it contains many references to a number of thinking concerns, thus I believe you need to read at least one of his others, of which I believe none are better choice than Lateral Thinking.
Why I recommend Six Thinking Hats:
Reason 1: Increase in productivity, both my own, and during team based activities such as meetings or workshops.
Reason 2: I, and many software industry people I have met, pretty much rely on a single very useful and relevant style of creating solutions for problems - namely extreme pragmatism. This works well, but is very limited for a future vision, and business people (aka clients) generally are more interested in the future and the environment created for their people.
Reason 3: Increased knowledge of how I think and how others think is invaluable as we are all knowledge workers - aka thinkers.