You can't book accomodation in the albergues. It's whoever arrives first gets a bed - not necessarily blankets, so yes, you need a sleeping bag - the next lot who arrive get a mattress on the floor - somewhere in the building - the last lot may get the floor, so you're best to bring a Thermarest mattress with you. Not all albergues are as bad as this but you need to be prepared for what it MAY be like.
Sorry if this is depressing news. Sadly, pilgrims are leaving earlier and earlier in the morning in order to get a bed for the night.
Of course, there are hotels........