I read this book when it was first published (and did the same with its sequels). I loved it but didn't bother writting a review to explain why, at the time. Having come across what other reviewers have to say about it - and it's rather negative - I felt compelled to explain what I liked so much about it.
First, there is the historical context: the First Crusade, and more precisely the winter of 1096-1097 with the various armies of Crusaders camped before the walls of Constantinople and requesting food and passage across the Bosphoros from Alexios I, the Byzantine Emperor. Now Alexios had initially called on the Pope to send him a few thousands mercenaries knights with which he could reconquer Asia Minor for the Empire; Instead, he got several armies of tens of thousands which invaded, poured across the Balkans, campred before the walls of the City, and didn't want to recognize him as the natural leader (since he was the Emperor) of the Crusade. Needless to say, Alexios doesn't feel inclined to feed potential ennemies and even less inclined to let them into his capital. This is the book's background, full of tension and where any incident could lead to war with the Frankish armies growing both larger and hungrier as new contingents arrive. Historically, it is perfectly accurate.
Second, there is the plot to assassinate the Emperor Alexios and this is where our byzantine detective "truth-teller" Askiades comes in: he is commissioned by Krysaphios, a high ranking official from the palace, to uncover the truth. The attempts to kill the Emperor are particularly interesting: one using a crossbow, another by black mailing one of his Varanguards into trying to kill him with his battle axe. Of course, our hero uncovers the truth. However, throughout the novel, nothing is exactly what it seems (hence the book's title).
So, I can understand and appreciate that this may not be the best of all thrillers and I would also agree that, at times, Askiates' family life, relationship with Anna, the byzantine doctor and supposed big secret from his past (which we never learn about in fact) feel a bit tedious. Also, the plot fits in very well with the historical background, contrary to what other reviewers have mentioned. Moreover, what some have presented as "cliches": Barbarian crusadeers = bad and Byzantines = good is simply disingeneous: Just read the book and you will see that it is a bit more suttle than that, and therefore Asliates' inquiry gets that much more dangerous for his own personal safety.
Anyway, I very much liked this book, if only because it is one of the rare ones that precisely does not always paint the Byzantines in a bad light and tends, for a change, to also show the Byzantine point of view and not only the Franks' one. How would you have felt if, one fine morning, you had woken up to find several armies camping in front of your city and requesting food and passage as "fellow christians"? Rather unconfortable and weary, I suspect...