is this a standalone?
what other of his books do you recommend?
Gosh.
He started out - recommended by Christopher Tolkein (nephew of the legendary..) - as a sort of literary executor to the legendary J R R Tolkien (and, by all accounts, discharged this duty in an exemplary manner). For example, he is supposed to have worked pretty extensively on the preparation of The Silmarillion.
The upshot of this, inevitably, is that his earliest work - a trilogy - (the Fionnavar trilogy) is heavily (over heavily, to my mind) influenced by Tolkien. In fact, my personal recommendation would be to skip this - and its sequel (written years later, a book called Ysabel), completely. Personally, I didn't care for them, at all.
Most of his other work ranges from the extremely good, to the excellent, to the outstanding.
While the books range from country to country, era to era, there are a few grace notes - a kind of chuckling in-joke - which appear in some of the books, subtle references to stuff, people and places that have gone before.
This latest book - 'Children of Sky and Earth' - is, yes, a stand alone, but it can be seen as a 'long' (very long, we are talking about about thousand years, approximately) after - a not quite sequel, of two earlier books - (which might have been but never were a trilogy), 'Sailing to Sarantium' and 'Lord of Emperors'.
In essence, Children of Earth and Sky is set in an alternative Venice, Dubrovnik, Prague, and Byzantium 25 years - and a few added - after the Constantinople/Byzantium of that world fell to the Ottomans. The real Constantinople fell to the Ottoman Turks in 1453 (no dates are mentioned in the book) but - given that, with a bit of historical licence, we are talking about an alternative Adriatic and related areas circa an alternative 1480, it is a very nicely taken interpretation and/or version of history.
Sailing to Sarantium, and Lord of Emperors were both set in an alternative (and superbly sketched) empire of an alternative Constantinople in the reign of an alternative Justinian (roughly 400 AD/CE). Two brilliant books, I loved them.
Actually, I was surprised, and delighted (and it wouldn't have been picked up on if you hadn't read the earlier Sarantium books - closely) - some of the reviews (erroneously) referred to other works as an inspiration, but the Sarantium books offer some of the cultural - and fantastical (even if exceedingly subtle) context for this work.
Now, you don't have to have read them; but, if you have, you will enjoy this book just that little bit more, as a few of the - almost asides - will make a bit more sense.
So, my preferences: This is something, which, by definition, is very subjective.
I loved the two Sarantium books (Sailng to Sarantium, and Lord of Emperors - this is a story in two books; if you like Byzantium/Constantinople circa the reign of Justinian, this is wonderful stuff).
Others I really liked were 'Tigana' (a kind of alternative Renaissance Italy, with possibly the strongest fantastical element in almost all of of Guy Gavriel Kay's oeuvre); and 'A Song for Arbonne' (a take on an alternative Provence at the time of the Albigensian Crusade).
Now, while I liked The Lions of Al-Rassan (his take on medieval Spain, about to kick out his alternative Moors), it was my least favourite of his own (as in not Tolkien derived) works.
The Last Light of the Sun is a strange work, set in an alternative Viking plagued England, one ruled by an alternative Alfred the Great. I thought it gripping.
The two Chinese (alternative history) books are also excellent, but require close attention. They are Under Heaven and River of Stars (and the events of, and the world of, the second of these have been significantly influenced by the events of the first, which occurred a few centuries earlier, in the same world).
I think that you would enjoy them; they are intelligent thoughtful, thought-provoking books; the world building is excellent, the characters - one of Kay's strengths is how he gives his secondary characters credible depth and backstories - engaging, the female characters extremely well written, and the story, a beguiling version of a history that might of been, could have been, or never was.