Rest assured that you are not the only one to bring a notebook or similar items on Camino. The Camino is a thing full of humanity from the best to the worst and everything in between. As someone said above, if there is a someone that is desperate to steal an item, it will be stolen. However, there are not a lot of those kinds of people on Camino. You will not need to bring a 2,000 pound safe requiring it being bolted down every evening to protect your valuables. However, you do need a modicum of common sense. Don't leave it lying about to tempt others or to lure a burgeoning criminal access to it. For me, I would keep it in the bottom of my backpack by my bed. However, I understand that this world is made of all types of individuals. Some live in a state of constant anxiety where they fear something bad will occur at every moment. Others don't worry as much; they take sensible precautions and realize that sometimes bad things will happen. If you need to bring it, then bring it. If you don't need to bring it, then leave it at home. You will not be the first person that has brought a similar item and returned home with it. Spain is a great place full of great people. The Camino is not a den of thieves, but just humans where most are on pilgrimage. Don't worry about it and enjoy your Camino.