First of all, it is best to read Ouspensky in english, because it is the original version, and one gets it first hand.
This is one of the most complexes books ever written about the Fourth Way, Gurdjieffs teaching. Ouspensky was Gurdjieffs best student, and in this book, he gives further all his understanding, by answering to all possible questions, that various aspirants ask him. Even if the book is written quite intelectual , he stays practical, and turns everything back to the same point : stay present, go out of imagination, expressing of negativity is harmful for your inner work, dont talk about things you have not verified yet. And even if these things seem to be so simple in theory, he knows how to practically work with them and brings you out of complicated thought situations.