I was prompted to write a review for this book having read two others with which I (both) disagreed. Both were emotionally-charged responses that were inaccurate for different reasons.
Robert Spencer has a problem: on one hand, he is very well-informed and does present a great deal of useful, sometimes shocking information; on the other hand he can be too aggressive in his approach, usually constructing identities of "us" and "them", "east" and "west". As a (practising) Catholic myself, I found this book to be worth my time, as Mr. Spencer clarifies things with which Catholics are familiar (such as the Virgin Birth) in relation to the Islamic position. A point that Mr. Spencer makes that is really the focus other books (which I would not necessarily recommend) is that a practising Muslim has good reason, based on Koranic principles only, to be a militant Jihadist. Also, the most devout Muslims, who take their faith seriously, are more likely to turn to violence, as the Koran (to a lesser degree, Hadith) is the only source of instruction relevant to personal moral conduct. I have read a fair amount on this subject and it seems that Mr. Spencer neglects treating Muslims who disavow certain interpretations of the faith. Spencer might argue that the consistent Muslim would have to concede that there is only one interpretation of the faith; others--less aggressive--are simply heretical. This may or may not be true: that is for Muslims to decide, not me.
To Spencer's credit, he does point out many inconsistencies and contradictions in the Koran, which may have offended the reviewer engaged in "interfaith dialogue". To some extent, I think non-Muslims have to be cautious about being naïve in their assessment of Islam: the desire to find common ground must not be done blindly. We don't know very much about Islam. For example, we consider ourselves (Christians and Muslims) monotheists both, Muslims consider Christians to be polytheists. Spencer tells the reader why. Also, Spencer explains why Muslim cultures have certain views which we Westerners find unacceptable in our own culture (e.g., treatment of women, attitude towards Jews). Of course, these things--as unsavoury as they may be to the West--are part of life in the Middle East. Once again, such information may offend those who choose to overlook differences and focus only on very broad connections between Christianity and Islam.
The question-and-answer format of the book makes is very accessible, though Mr. Spencer's writing is full of sarcasm and attempts at wit that may go over well with some, but seriously missed the mark with me. I would really like to see a similar book--written for Catholics and Westerners in general--that lacks the sarcasm. Despite his attitude, it is clear that many of his points are valid. When Western authors write about Islam, it is really difficult to know who is presenting accurate information; maybe therein lies the difficultly. Catholicism's faith is comparatively easily presented, as there is only one accepted doctrinal position, centralised and organised. In Islam, there is only the Koran, Hadith and individual Imams (the structure is much like most forms of Protestantism).
In Europe and America, there are a different set of problems to approach than in the Middle East, where Christians are the minority and are really persecuted. Although somewhat controversial, a Coptic priest is very popular in the Middle East, explaining Islam (can be seen on YouTube, Fr. Zakarias Boutros) is being hunted by Muslims. This is possibly the greatest problem with people who seek to reconcile Islam with the West: how can freedom of religion be practised when Christians are not allowed to speak openly and Muslims can be killed for leaving their faith. I continually hear "there is no compulsion in religion", but clearly there is. For those who speak of interfaith dialogue, please do so in Saudi Arabia, where Christian Bibles are prohibited, much less Christian Churches (this has been the case, and I assume nothing has changed in 2010). I have a great deal of respect for my Muslim friends, one of which is very close to me; however, it is difficult to talk about things I consider to be basic human rights as some Muslims (at least governments) disagree with Christian-based ideas of freedom and charity towards all, even sinners. The problem with questioning the Muslim faith is that the Muslim faith cannot be questioned. I don't expect Muslims to all become Catholics, though I would like to see all Muslims embrace peaceful co-existence with all members of society. (Of course, this means Christians should do the same, especially certain countries, like the U.S., which only provoke the Muslim world).
In short, while I didn't care much for Spencer's writing style, I think this is one of the few books of its kind. It does a reasonably good job of explaining the basis of Muslim belief to Catholics and other Christians. I might avoid this book because of Spencer's attitude, though I don't know which book I could recommend in its place. For a much more positive interpretation of Catholic/Christian-Islamic dialogue please see
Ecumenical Jihad by Peter Kreeft.