Well marked, except when leaving Deba: a local land owner changes the arrows around so pilgrims will get lost. So in Deba, stop at the tourist info office and they will give you a handmade map with correct directions.
Mostly asphalt I'm afraid. And the few bits that aren't are steep, going up and down ;0) Do bring walking poles. If not super limber, avoid going over the Brusco as you come into to Noja, walk around. As beautiful as the views are, that is the one and only place I have ever felt was dangerous on my Caminos. Lovely ravine for you to fall backwards towards the surfers below.
Experience: the food and drinks! Aim for a mid-afternoon lunch and order Menu del dia. Gorgeous food most times, certainly much better than the CF Pilgrim menu. Plus, if you don't make that meal your main meal, you will find yourself eating sandwiches at night (bocadillos). Try the local Txacoli (sparkling wine), cidra (cider) and vermud solera (aged vermouth). For favourite meals: fabada (bean and meat soup) also called something or asturiano stew and has another name. Also grilled sardines. Anchovies in Santona. When in San Sebastian try two of the most well known restaurants for their pinxos (tapas): La cuchara de San Telmo and A fuego negro. Both on the same street in the old part of town, but La cuchara has its entrance on the back of the building, to the right of the little plaza. In Bilbao, in the old part of town, there is an inner courtyard with lots of pinxos restaurants all around it. Lots of wonderful food to sample. In llanes, indulge in Carbayones, a lovely local desert.
Tip, in Bilbao, do NOT stay at the albergue across the river from the Guggenheim, the Botxo. There is no security at night. Too bad because the location is fantastic. Impecable albergue, 50 meters from the train stop in Mogro where you should take the train to cross the brige: Casa piedad - duvet, bed made, towels, shampoo, private bathroom. In Noja, stay at the hotel la Casona - they have a few rooms for pilgrims at 10 euros. Mine had two single beds and its own bathroom with bathtub, and since noone else showed up I had it all for myself and for 10Euros. In on the main plaza, next to the church, restaurants and tourism info center and certainly beat the local muni on the way into town whom I've heard is falling apart. Also, do NOT stay at the albergue in San Vicente de la Barquera, but do go up the hill to the fortified part of the town and visit the church up there: it's one of these churches with such a special feel, and historical relevance, like Eunate on CF. In Santillana del mar stay in the pilgrim section of the Solar de Hidalgo, a magnificent manor house as you enter the older part of town, on your right. Pilgrim rooms in the back, but to get there you walk through 100s of years of history. Just skip the 4Euro breakfast. And in Santiallana have dinner at El pasaje de los nobles - little chabby chic place with a lovely menu. Has entrances on 2 streets. Finally, as you leave Guemes, aim for the longer route along the cliffs and not the inner route, along ... cars ;0)
As for guides, si hablas espanol Eduardo, why not go for la Guia practica del
Camino de Santiago: Camino del Norte (Costa y primitivo) from Editorial Buen Camino. If German is up your alley, or if you just want very detailed maps of the area, and not just a sketch of the Camino path itself, why not try the Rother books.
Have a wonderful Camino!