After Such Knowledge Trilogy, each volume is a different genre: historical fiction, fantasy and sf, respectively. However, each volume is also a distinct narrative. The Trilogy is thematic, not linear, three related works, not one work in three parts.
By contrast with this, Poul Anderson's "Star Of The Sea" seamlessly incorporates four kinds of writing into a single narrative. There are:
people living in the past (historical fiction);
time travelers based in the twentieth century visiting the past (science fiction);
successive stages in the development of mythology (fantasy);
a concluding prayer to the Virgin Mary, who succeeds the goddess (prayer).
The alternation between genres begins in the concluding sections of the previous installment, "The Sorrow of Odin the Goth":
time travelers in Hawaii in 43 AD (sf);
the death of King Ermanaric (historical fiction);
a divine marriage (myth/fiction);
the German siege of the Roman Old Camp (historical fiction);
Manse Everard arriving in Amsterdam in the late twentieth century (sf).
A German prisoner says that Rome is doomed, according to the goddess. Everard arrives in leisurely wise centuries later but nevertheless, thanks to time travel, will do something to preserve Roman rule in Northern Europe.