As a former nun, who has clearly rejected the Christian theology of Trinity, Karen Armstrong emphasizes and eagerly embraces Islam's monotheism, its universalism and its charity. But that's about it.
You won't find an open discussion of the faith, its history or its faults here. Islam means submission--in other words, making others submit to the will of Allah. Armstrong fails to cover that aspect. You will not read about early Islamic massacres of infidels, which Armstrong glosses over, as if they did not occur.
Nor does she deal with Islam's historic persecution of peoples of myriad other faiths, including, but not limited to, Zoroastrians, Ba'hai, Hindus, Coptic Christians, Sudanese Christians and Middle Eastern Jews. The chauvinism exhibited by Mecca, a city closed to all but Muslims, is similarly avoided in this (mercifully) brief, overly sugared pabulum.
For brief lessons on the faith, one is better off turning to the Internet writings of Muslim believers like the Shaykh Prof. Abdul Hadi Palazzi, who heads Italy's Muslim community. A holy--and moderate--man, he shares the rich beauties of the faith and discusses the meanings of key passages in the Qu'ran and Hadith. He also openly discusses how badly Islam has been perverted by the Islamic sects and leaders controlling more than 75% of the mosques in the western world.
For history, Bernard Lewis provides work revered by Western students of the faith and Arab and Muslim scholars alike. Islam in History provides a sound basis for understanding the faith and its place in history; Islam and the West and The Muslim Discovery of Europe are equally informative.
For an understanding of Islam's political effect on other peoples, one can turn to the scholarly works of Bat Ye'or-- Islam and Dhimitude: Where Civilizations Collide or The Decline of Eastern Christianity Under Islam: From Jihad to Dhimmitude. The latter covers the 7th through 12th centuries.
But give Karen Armstrong a pass.