Except the history of earliest phase of Islam, this is a great book to read as an introduction to Islamic history. Its short and well written as all books by Karen Armstrong are. At one place she states that ‘the claim that Prophet Muhammad had been the last and greatest of the prophets is not made explicitly in the Quran’ . But if you see 33:40 of Quran it is stated that Muhammad PBUH is the seal of the prophets. Apart from this and some other mistaken details , this book is worth reading.
The best book on beginning of Islam as a biography of Prophet Muhammad PBUH is that by Allama Shibli Nomani and his student Syed Suleiman Nadwi. For anyone reading the biography of Prophet Muhammad PBUH for the first time, it is highly recommended that it should be read. The author clearly indicates why western historians and biographers make mistakes in recording the life of the prophet. One wonderful example is how the writer has proved that the Battle of Badr was a defensive act by Muslims. It is a 7 volume text in Urdu and I luckily bought all volumes for 1200 rupees from new Darrussalaam shop at University Road Karachi.