The first book sees the aging mosaicist, Martinian, summoned by no less than the Sarantine Emperor himself to journey to the capital city of of Sarantium to assist in a massive building project there. He doesn't particularly want to go, and he manages to persuade his partner, Crispin, to assume his name and journey in his stead. The book traces his travels to the city, his involvement in political intrigue there, and his eventual start of work on the project.
What makes this a completely fascinating and engrossing book is that the world Kay has created is a retelling of 6th-century Byzantium. The Emperor Valerius II is an analogue of Justinian, Sarantium is, of course, Constantinople, and the enormous monument on which Crispin sets to work is none other than than the Hagia Sophia itself. Pretty much all the major characters of real-world Byzantium are included in one guise or another, from the actress who became Empress, to the great general Belisarius (Leontes), to the court secretary and writer of chronicles Procopius, who keeps his hatred of his Emperor concealed until a crucial moment in the second book. Tribonian, John the Cappadocian, and even Mohammed himself are part of the story. There are as well various imagined minor characters, and Kay's skill is such that they seem none the less real for their imaginary status.
The history of the fictional world Kay creates is our history. In the first book, he flashes back to the Nika riots, in which Valerius is persuaded by his wife not to flee the city, and so orders a military response which results in the butchering of 30,000 citizens. Part of the backdrop of the first book is the religious discord between the pagans from the western, fallen parts of the Empire, the Orthodox, and the monophysites.
Kay takes this historical grounding and adds his own fantastic elements, his 'alchemy,' which isn't alchemy in the real-world sense of fuddling around with reagents, but is a darker, more dangerous affair of mingling reality with what is termed the 'half-world.' This is a subtle underpinning, however; while it is crucial to plot development in places, characters do not go around wielding spells and magical implements, and the only fire being summoned forth against various persons is of the Greek variety.
The second book details further Valerius's plans and attempt to reunite the sundered halves of the empire, as well has his designs on the Persians to his east, and demonstrates why they are doomed to not be realized. Crispin's efforts to finish his mosaics in the temple are ended, if not completed, and governments rise and fall.
I think Kay's quasi-historical approach is a brilliant one, because it simultaneously allows us insights into the nature of the historical period, places, and people, and allows our prior knowledge of the real-world equivalents to inform our concepts of the characters. Valerius II, Alixana, and many other of the principle characters wield vast power over a tremendous area of the world, can order the deaths of thousands with but a word, and yet Kay makes it possible to empathize with them. To paint people who are at the pinnacle of human society as characters with depth and substance, while managing the scale of things well enough to give the same treatment to Bodo himself is a rare thing in literature, and even moreso in the genre in which Kay writes.
These are tremendous, moving, emotionally involving books. The first moves slow in places, as exposition tends to, but even so, when the book comes to an end, it does so appropriately - too often first books just run into a brick wall and end abruptly, which makes sense when you consider they're just the first part of a single larger work that was split into two more-or-less arbitrary parts for ease of publishing and marketing. But Sailing to Sarantium draws gracefully to a close; certainly not all matters are resolved, but the ones that are left open are left open in a pleasant fashion. Then the second book comes along and knocks your socks off.
To borrow a phrase, I wish I had two hands, so I could give them four thumbs up.