This is an old book now, and the first I read many years ago on Arab Middle Eastern culture. It was at the time an inspiring and fascinating book to read, giving a precise breakdown of the term Arab, and disentangling the many diverse strands that underlie this homogenous term. Lewis looks at the arts, literature, scientific and cultural developments contextualising them within their cultural frames. In all sorts of respects this is a good primer for those looking to find a readable introduction to Arab histories and cultures.
In terms of deep analysis i think this is where Lewis falls down. He tends to adopt something of the Orientalists posture. For a good and very exhaustive definition and more important, analysis of 'Orientalism' look at Edward Saids book Orientalism.
Having read most of Lewis's books, I would say this is his most enduring contribution. The post 9.11 books on Islam are quite disappointing for a serious scholar, although in his defence, no doubt he feels a measure of despair for what he has seen develop in the Middle East, and this translates into judgement and biased commentary, which this early book was markedly free of.