We have just returned from Santander too! We were amazed bu the good value of the Feve trains - two of us from Bilbao to Santander for 15 euros. It did call at every tiny station and took 3 hours.
We stayed at the refuge on Sunday night. You go to the Bar (called something like Beruca) in the Ruamayor and they take take you to the refuge. They were friendly and didn't mind us turning up at 10.30 pm, but quite firm! I don't know if they have credentials, but they do have stacks of good looking guides for the Cantabrian section of the Del Norte for sale.
I would recommend you get something a bit more user friendly than the Eric Walker's guide, in whatever language you can find! And a good map, if you like to know about the contours and place yourself in the surrounding area rather than just follow instructions blindly. We were on bikes, and had the Michelin 1:150,000 which was not good enough.There is a book shop in Santander called something like Estudia opposite the cathedral. (They may still be celebrating the enormous purchase of children's books we made - why did we ever stand the tradition of buying the grandchildren books in the language of wherever we have been? ) It had plenty of 1:25,000 maps, but no 1:50,000 that we could find. It did have other maps which may be useful.
We found the Norte quite hard going for bikes with lots of embarrassing walking up hills. We are thinking about continuing on foot (it will be from Gernika which was where we had got to when my legs said No More!) because that the route goes across hills and valleys and through villages while the roads go up and down very different routes, and we missed out on lovely sounding villages because we were worried about making it to the next refuge.
Have a great time!
Bridget