Background Reading for Camino Del Norte?

I don't know of anything so specific. Laura Perazzoli and Dave Whitson's Pilgrim Route: The Northern Caminos (Cicerone) has a fair bit of material scattered about in its pages, usually in pointing out a building of interest. Hugh Thomas' The Spanish Civil War has several chapters about the prelude to the war in the north, particularly Asturias and the Basque country, and is a good read (Lord Thomas was from the generation of historians who wrote to be read by non-academics). An entertaining and informative book about the Basques is Mark Kurlansky's The Basque History of the World. For some background on modern Spain, Giles Tremlett's Ghosts of Spain is worth reading.

I recommend Foders or any similar travel guide --you will pass through a lot of tourist areas and the travel guides have lots of advice on what to see, when it's open, places to eat/sleep, and a bit of history/culture. There were some interesting churches I'd never have seen if all I used were Camino guides.

This should help you with the first part of your Camino del Norte trek:
https://www.amazon.com/dp/0140298517/?tag=casaivar02-20
I included the amazon link because you can pick up used copies pretty inexpensively, but recommend borrowing a copy from your library, in case you don't think its a keeper.
Buen Camino!