Entymologists may have a take on this that I don't, being a mere natural history buff. I actually read the first 250 pages straight through (in bed - no mean trick with this 10 pound item), then browsed the rest for information, returning to steady reading at the end. The beginning chapters give a history of insect studies and go at some questions of general interest (what was the evolutionary history of the development of insect wings, for example... fascinating stuff). The final chapters deal with general questions of modern insect history and the future in our increasingly screwed up planet. Both the beginning and end of the book could be read usefully as essays in themselves. The great middle bulk deals with the evolution of the particular families and orders, and is,again, full of interesting details (no, the modern roach doesn't go back to the Carboniferous, no matter what they say in the Exterminex commercial!)
So the technical (but perfectly clear) middle five hundred pages becomes for me a once-scanned part of my reference library to which I'm sure I'll be returning many times. For me, not being a trained biologist, one good feature of the book is that I got a - hopefully partially retainable - idea of the various ages and epochs in geological time (how many times have I tried to get those down?) and a general notion of the various features of groups of insects and their relationships to one another. I think it's much easier to retain this information in the framework of evolutionary history than from a field guide or books on specific groups. But ask me again in a year. Of course, the book is very well illustrated and generously sprinkled with graphs and timelines.