Vikings The Italian Saga

In the year 860 a fleet of Viking ships penetrated the Mediterranean. After plundering coastal cities in Spain and France, commanders Hastein and Björn Jaernsida ("Ironside") decided to head for central Italy.
Having reached and raided the port of Pisa, the Vikings sailed up the Arno River and arrived in the territory of Florence and Fiesole. The "city on the lunate hill" and the castle of Bishop Donatus were conquered. So Florence was also besieged....
The poverty of medieval sources does not allow us to get into the heart of these events, but the author, with investigation in real places and with a little imagination, tries to fill in the gaps to make the historical circumstances understandable.
We are talking about a fact that is as little known as it is surprising and significant: the daring expedition carried out by those northern sailors, rough men, scornful of danger, who later knew how to participate as protagonists in the cultural development of Europe.