Hring Kristjanson, son of a the thegn of Long Isle, has been cast out of his religion (Christianity) and his nation (Treeland) for the crime of killing his half-brother (which he did, somewhat accidently, when filled with blood-lust during training). The book is something of an epic fictional biography of a man and his actions in North America, starting in 1485 when he is exiled.
Roberts has created an alternate world which breaks from our own time-line in 995 AD. In 995 AD, some pagan Norsemen, refusing to be converted to Christianity, set sail and eventually arrive on what would later be named North America in our time-line. They establish a country of their own (which they call Treeland), stretching, mostly along the coast, from our Maine to our Virginia. Later pagans follow (including some fleeing Ireland). Then in 1066 AD, William the Conqueror invades and captures England. Instead of buckling under to his rule, the Christian Saxons flee to America and Treeland. After living together for less than roughly 100 years, the pagan south and the Christian north split into two separate countries, with the Potomac as the boundary (the Pagans take the name of Thorsheim and the Christians retain the name of Treeland). About five years after the split, a new settlement and kingdom is created in our Florida by Muslim Spain (not fleeing Muslims, but colonizing Muslims from Muslim Spain). Meanwhile, the Aztec Empire continues to be very powerful and mighty, maybe the most powerful empire in the North America of 1485.
Those that have found a paperback copy of this book, maybe the first Tor printing of April 1986, might be confused by the blurb on the front cover: "Vikings clash with Mongol hordes and America is the prize!" The blurb is very misleading, and mostly incorrect. I don't want to give too much detail away, but will note that the Mongol Empire does play a factor later in the book.
The book is very well crafted, the plot is strong, where other authors would spend hundreds of pages describing certain activity, Roberts quickly, effectively, and nicely conveys the same information in a much smaller number of pages. The characters are very well-developed. The description of the Aztec Empire and the Mongol/Khan invading army is very good, while the Treeland, Thorsheim and Muslim Empire is a lot thinner. The Incan Empire is barely touched on, just a very basic mention, without noting any names (empire on the southern continent). I don't recall any magical creatures or the like, though the pagan religious practices described in the book, I suppose, move it into the Fantasy genre. The violence and descriptions of nudity and like activity definitely move this book out of the realm of books children should read. At the very least, the book should be treated as rating PG-13. Overall, I would give the book 4.38 stars.