While this book provides a wide survey of the middle ages, it shows some lack of objectivity and use of proper historical evidence, especially regarding Arabs, Muslims, and Islam in general. I would not have paid attention to such a superficial dealing with historical facts and characters if it were from a non-historian, but the authors are specialists and have seemingly done their research with other historical characters and facts (have they?!) It does cast shadows of suspicion on the other parts of the book to see the authors describe Haroun Al-Rasheed the Abbasid Caliph as 'the one we know from Arabian Nights'! I thought it was common knowledge that Arabian Nights is a purely fictitious book and that the Haroun Al-Rasheed in it has nothing to do with the real one, yet one still lives to see. If this is what the authors could present as a description of the Calif whose reign had witnessed the climax of prosperity and civilization of the Abbasid Caliphate (ironically, as per the authors' comment!) then it casts heavy shadows of suspicion on the validity of their research; and this is not an aggressive generalization because this incident represent a single example among several.
Of course each historian's work carries his point of view and his inclination towards certain theories and explanations, but when it comes to research then the mere simple truths are threatened to be obscured, if not utterly twisted. One then has to take heed when reading this book and seek more objective and well-researched sources.