This series, while not as strong as some of his earlier works, such as The Lions of Al-Rassan, Tigana, or A Song for Arbonne, is still characteristically Kay. Like all Kay books, the two in The Sarantine Mosaic feature an epic scope, richly detailed settings, complex and well-developed characters, tortuous plot development, and a pseudo-historical setting; in this case, a fantasized, slightly magical version of the city of Constantinople (Sarantium) during the reign of the Emperor Justinian I (Valerius II). I'm not going to give a plot summary because if you want a general overview of the events in the book, you can simply read up on the life of Justinian I. Kay only maintains historical accuracy when it suits his purpose, however, so browsing Wikipedia (or whatever other source you choose) won't reveal any secrets.
If this sounds like a lot to pack into two books, it is. The Sarantine Mosaic is a demanding read, with every one of its over 1,100 pages packed with information. None of the characters, descriptions or events are throwaways, and anyone who thinks otherwise will soon be kicking himself for not paying closer attention to the description of the Emperor's handwriting style when it becomes important 300 pages later (not an actual example from the book, but similarly minute items are given prominence quite frequently).
Kay does a good job of giving the reader subtle hints and reminders when he references events or people that haven't been mentioned for a few hundred pages, but it is still tough to empathize with all of the dozens of characters comprising his cast, and that makes some of the poignant moments fall flat. Kay forces you to pay so much attention to the smallest details of his writing that sometimes the emotional effect of his text is lost as you try not to miss significant word choices.
I want to let Kay speak for himself a bit here, because the back of Lord of Emperors contained an excellent essay by Kay on his writing process and his views on the fantasy genre, which I think it will be helpful to share:
"Fantasy has never been in its essence about constructing elaborate magical systems for dueling sorcerors or contriving new versions of an enchanted ring or further variations on the use of hypens and apostrophes in invented names. Fantasy is—at its best—the purest access to storytelling that we have. It universalizes a tale, it evokes wonder and timeless narrative power, it touches upon inner journeys, it illuminates our collective and individual pasts, throws a focusing beam on the present day, and presages the dangers and promise of the future. It is—or so I have argued for years—a genre, a mode of telling that offers so much more than it is usually permitted to reveal." (560)
Kay's writing perfectly matches his views. When the current Harry-Potter-induced fantasy craze ends, and all the books written by 15-year-old kids who were following the "sorcerers + magic rings + weird names = bestseller" formula that Kay denounces are forgotten, mouldering in dusty basement boxes across the country, Kay's books will still be on bookshelves, well-loved and well-read.