The Sarantine Mosaic is a duology of novels by Canadian author Guy Gavriel Kay. The titles of the books that make up the work are "Sailing to Sarantium" and "Lord of Emperors".
Set on the same world as his standalone fantasy The Lions of Al Rassan, (these books are not connected, they just share a setting) the novels present an alternate history view of The Byzantine Empire under the governance of Justinian, centreing around a mosaicist, Caius Crispus, who is called to Sarantium, the capital of the empire, to work on the dome of the emperor's new church.
Like all Kay's novels, the story is painstakingly researched, and the detail is highly authentic, from the mechanics of creating a mosaic with glass tesserae, to descriptions of chariot racing, outlines of factionalism within the empire, religious development, and the effects of plague. While the books are nominally fantasy, the magic they contain is almost incidental, and there are none of the stock elements -- no omnipotent magicians, no battles between dark and light powers, no quest, no evil villains.
Instead, the plot is a complex weaving of internecine and personal rivalries and conspiracy, together with in depth character building and portrayals of relationships.
The language Kay uses is flowing, educated and full of really beautiful imagery, and he creates characters the reader really cares about in a plot that fascinates.
This is a story that transcends genre, and is equally accessible to fantasy lovers, students or readers of history (especially Byzantine history), or those who read for the delight of good characterisation, eloquent prose, or masterful plotting. It's not mind-candy, and requires concentration, but it really is worth the effort.
I would recommend it. Wholeheartedly.