To read this book effectively, you really need to have the articles that the authors cited in the book, especially their own articles.
The ideas from the book are excellent. However, the presentation of the is extremely bad. I am not saying for just myself. Many of my professors and my peers also agree.
The authors skipped a lot on the derivation of the equations without mentioning where they came from, or without even defining some terminologies. Sometime, you will get really frustrated.
I prefer to learn not just the ideas but also the technical foundations. Barro and Xala-i Martin's book on Growth is excellent in this. It is impossible to learn that in this book. Sorry, the book is a failure!!!
I talked to my friends in other schools in Cambridge. They were surprised because Aghion is supposed to be a very careful professor. I don't know. But I have attended a conference and seen how Howitt presented. It is a big, really big, conference, and the guy just wrote down those equations and calculations by hand. They are even not neat.
The exercises are interesting and helpful if you want to master the tricks. However, again, you need to read the articles that they used since many steps will be skipped carelessly.