Specifically about Rollo and early Normandy?
Depends. Can you read french?
-
Le monde franc et les Vikings (VIII-Xe siècle) or
Des raids scandinaves à l’établissement de la principauté de Rouen by Pierre Baudin.
The first is quite the key work, less about Normandy and Rollo than a general outlook on the relations between Vikings and Franks, something quite related to.
The second is an article part of
La Normandie avant les Normands, and more focused to your OP, but the book is essentially an universitarian study so you would have better chance checking an university library or asking them to order it.
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La Normandie des ducs aux rois. Xe-XIIe siècle or
L'aventure des Normands, VIIIe-XIIIe siècle (maybe more accessible when it comes to reading, but less avaible giving it's a book from a collection) by François Neveux (the authority on Normand history)
- If you don't mind order by internet,
Rollon chef Viking may be interesting (I never went into it or his author's works before that said).
It seems to be illustrated.
If not...I don't think that I know a good book. There's maybe translations, but they are kind of uncommon, critically for newer works.
- Dudon of Saint Quentin's chronicle (
De moribus et actis primorum Normanniae ducum) is heavily hagiographic and should be read cautiously, but it's a primary source.
In english
-Any John Haywood is probably worth of consideration
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Historical Dictionnary of the Vikings is interesting and if its informations are basic, they are trustworthy and have a good bibliography that would probably help you more than I.
As for the what if, could Rollo had converted to Christianity but kept the Nordic law and customs intact if he had spent since 885 keeping Rouen and the area between it and the sea solidly under his control?
Rollo was a raiding chief : when he came first in Neustria, it wasn't for settle down but to raid.
He participated to
*this* Siege of Paris, then to the Siege of Bayeux, making himself a reputation and making ties.
When he goes in England by the late 880's, he's not holding territories yet though, and it's probably this reputation (both on local nobility and among local Danish settlers) that allowed him in 898 to make a treaty with the bishop of Rouen.
The question is less if he was able to do that but why he would have done so. Conquering cities, ignoring the local powers would have been no that different than what Ragenold did in Brittany. So he could have.
It would have backfired, in a similar way. He would remain a glorified coastal plunderer racketting and blackmailing the neighborhood. Neighborhood that had the will and capacity to defeat him.
See, being christianised wasn't just a private conversion but a public acceptance of what you could call a "way of life", including politically and socially.
A conversion for the show, but changing nothing to your behavior wouldn't do anything.
More importantly, could Normandy stay with the laws or not?
Don't forget that Normandy customs (that aren't laws, especially giving their unwritten and non-permanant nature, but represent the main legal structure in middle-ages.) were maintained up to later times, and that they included legal legacy from Scandinavian customs.
Technically, while after much evolution, the
Normandy custom is still living on in the Channel Islands.
If you're asking for a more scandinavian-based custom, however, it's going to be hard, would it be only for demographical reasons (Scandinavian settlers being outnumbered, Frankish elite remaining largely in place, traditional quick adaptation of Norse settlers as everywhere they went)
As far as I know in Normandy, there were proportionally less Norse* settlers there than there was in the Danelaw (especially what was to become Yorkshire).
I think it's less a question of numbers, than being about :
1) Demographical importance relativly to natives : Northern England was underpopulated compared to the southern part. Norse settlement was more "obvious".
2) The political structure. Anglo-Saxon kingdoms had a more loose grasp as a political entity on their territory than what existed in Carolingian Francia, in spite of the troubles (would it be only the narrow net of cities). It's easier to establish your domination this way.
That would explain that in a century after the Norse settling in Normandy, they become wholly French.
I'm not convinced that Norses didn't assimilated eventually among the Anglo-Saxon population (even if in this case, they influenced just as much they were) : Anglo-Dane population often had AS names for exemples, and the elites fit in the Anglo-Saxons institutions relativly easily.
The wars with Wessex, and the creation of an English identity defined as "definitely not Scandinavian" shouldn't hide that.
* Also I believe the makeup of the settlers was different with more families than single men settling in Danelaw than Normandy.
I'm not sure of that, critically giving a really important part of Scandinavian settlement, in the pagus of Rouen for instance, was made from Anglo-Dane farmers.