I've read this book several times in a span of, I think, ten years - both in Italian and in English.
At almost 800 years after his death, Emperor Frederick II is still a fascinating and controversial figure, still admired here in Italy and a great protagonist of legends and outdated myths, so it's not so strange to find so many reviews in Italian, until 2010 it was easy to find this book in various libraries or stands and receives various acclaims.
As someone has already written, I could split this novel into two parts: the first part alone might be worth five stars, as a perfect example of good storytelling and with all elements that a work of historical fiction should have. Unfortunately, this is not the same thing I could say for the second part.
The break occurs with the born of Frederick's favorite child, Manfred, and the end of the Sixth Crusade: if the first part has a good balance between the description of the events, characterization of the protagonists, and narrative rhythm, the second unfortunately start to run too fast between important political and pivotal moment not only to Frederick's public life but also to the overall history of the time, dedicating to them only two lines if we are fortunate (considering how much importance was given to the context at the start of the novel, this detail seems completely forgotten at the end, creating incongruence with the author's narrative choices).
Instead, to me, the second part gives too much importance to the love story of Frederick and her (future) morganatic wife, which it's not a big problem if we consider that this book is, in the end, a novel, with some fictional elements and dedicated to the general public.
The problem is that in the pages in which we could have read a better or at least coherent explanation of important events (like the Siege of Parma of 1248) or the motives behind the actions of some characters (for example, the supposed betrayal of Pier Delle Vigne or the imprisonment of King Enzo, two highlights which were narrated in two lines even if had an enormous impact to the end of Frederick's reign, appeared in the story out of nowhere and without much sense given) we spend more than 50/60 pages to read the (truly pathetic) mental gymnastics of one of the most powerful men on Earth for a woman, which - historically speaking - all we have of her are posthumous legends: we don’t even know if her name was really Bianca, in which part of Italy and which family she was from, and most important where she is buried (fact: it's true that in Frederick's sarcophagus it was found a women's body, but has been certified that belongs to a girl in her teens or early 20s, who died very much time after the Emperor, so the story proposed in the book, although very beautiful, could be easily debunked. Also, it's hard to believe that Frederick, described also in supportive chronicles as an unsentimental and indifferent man*, could think to be buried with somebody in the same tomb, just see how he managed to bury his still beloved first wife in contrast to his grave) *(Matthew Paris and other contemporaries narrated only two times in which the Emperor could have publicly wept: the death of his son Henry VII and the betrayal of Pier Delle Vigne).
I found really hard to believe that Frederick, in the last years of his reign, when all of the work he created in his life started to crumble, used much of his mental and physical energy and time to cry about Bianca and we have to read it, know it, suffer for it, while the rest of the events are described in scarce details and the characters besides Frederick don't have the minimum of depth, first with Bianca herself (I was more sympathetic and involved by far with the first and third wife). Like, entire pages of fanfic with some glimpses of historical parts, in which major historical events that covered three-four years are described poorly, and we return to the fanfic part.
It's a shame if we consider that, for the first part - the perfect one, we have fictional pages with romantic plots mixed very well with historical details.
I think the author projected herself onto Bianca's pov, giving her much space and importance in spite of other female characters.
In the end, it's an enjoyable and not difficult read to a controversial and interesting figure of our 13th century, in which I think it's hard to find other fictional work, maybe because Frederick's life is pretty complicated to tell and, at least, The Falcon of Palermo try to offer its version.