Warriors of God is a novelistic treatment of the third crusade that is fairly readable, but does not stand up as a work of history. It is pacily written, but the author uses considerable artistic licence.
The fundamental problems lie first in the impression that the author has not really grasped the medieval period, something evident particularly in the early chapters, where there are some sweeping statements with no factual basis, and gross and inaccurate over-simplifications. Reston's use of modern terminology is more often misleading rather than illuminating. (In this respect, the book suffers badly in comparison with something like Rubicon, where Tom Holland did a much better job of conveying the essential strangeness of his period to a modern reader, while drawing parallels with today more subtly.) More serious is a failure to adequately explain the concept and attraction of crusading itself, something central to understanding the period.
The second key flaw is the author's determination to present a revisionist view of the period, with the Christians firmly established as the bad guys. The Christians were doubtless responsible for some terrible atrocities, but there is no real effort to explain why this happened or put it in the context of the time. The author presents the crusades as a perversion of the defensive concept of 'holy war', without any reference to the preceding centuries of rapid Islamic expansion. The line is at least set out clear in the introduction, where the crusades are described as 'a frenzy of hate and violence unprecedented before the advent of the technological age and the scourge of Hitler'.
I had high hopes for this, as I have studied the crusades and think it is good to bring history to a wider audience, but I was very disappointed. A wasted opportunity.