Probably the best of the series (the other two being Listening and Speaking). Provides a good framework for writing longer documents, covers tone quite extensively and has a good selection of formal business phrases.
On the negative side, I felt the book could have been structured better. There were also several errors. For example, "how" is not relative pronoun, as stated on page 25. Also, it fails to point out anywhere that the phrase "I would be grateful if you could...", which is repeated throughout the book, is not grammatically correct English, although it is widely used. The grammatically correct (and very formal) phrase to express a simple future condition like this is "I should be grateful if you would...".