Unlike most histories of the Crusades, which tend to be written from the Westerner's point of view, this book gives an unbiased perspective from both sides with slightly more emphasis on the Muslim view (hence the subtitle "The Flame Of Islam").
The information for the book has been gleaned from writings of the time, both Christian and Muslim. One minor criticism of the book is that it begins with the 2nd Crusade, so you will need to read elsewhere about the 1st Crusade where Jerusalem was captured for the 1st and only time by the Crusaders.
If your knowledge of the Crusades is a bit sketchy from the Muslim point of view, then this book will provide a good deal of information there.
However, bear in mind that this book was written between the two World Wars, and more information on the subject has come to light since then. Furthermore, the political map has also changed, what with the establishment of the state of Israel and Palestine, etc. All in all, an excellent read.