No cliffhangers; book 1 is a standalone, book 2 is a standalone taking place ** years after - I would not read any blurb in book 2 to keep book 1 spoiler free.
Book 3 takes place ^^ years later from book 2, and while I can see a general outline of where it goes from the so called "Encyclopedia of Nessantico" or something like that, dated 100+ years later and excerpted in book 2, I think it will also be a standalone.
In a way the Nessantico cycle resembles historical fiction a lot, with the city/country as the main character, while humans come and go though there are overlapping characters in book 1 and 2 and I am pretty sure they will be from book 2 to book 3
Edit later: actually Mr. Leigh explains it very well on his website:
"Imagine Renaissance Florence blended with late Imperial Rome and spiced with some of the trappings of ‘fin de siécle’ Vienna: a great city of a far-reaching empire; a city situated at the center of civilization; a city where art and music and writing flourish, where the world’s greatest minds and greatest talents come to make their reputations, while the trade of a hundred lesser nations passes through the ports and the streets.
That is Nessantico.
Consider a world where magic has for centuries been linked to a central, dominating religion and secret rituals, a world where magic is an integral part of the ‘technology’ of the world, but where now some people claim to be able to create magic without the rituals, without the religion -- people those in charge consider dangerous enemies. Magic here is created through a devout faith, and can only be performed by a special few whose minds are open to it. Here, magic lights the streets and powers the engines of economy; here, magic is the great weapon of war and forms the foundation of Nessantico’s greatness as well as the chains that bind its servant nations to it.
.........
As with the Cloudmages books, my intention is to make the books as "stand-alone" as possible. Each book will have a distinct beginning and ending, and the main characters will be different in each book. It is the city of Nessantico whose "character arc" will begin in the first book and end in the last, not the people themselves."
