Books - I usally have 3 (or more) going at once. But for the Camino, I wanted to be unencumbered and I also felt it was importnt to be engaged in the entire experience - no diversions. I went without even a guide book, no phone, basically only two changes of clothes, several bandanads (double as towels, etc) and a small journal.
In Leon, I bought a novel at a local shop. I quickly devoured itand left it behind. Other than that, I did not read. I was absorbed in life around me.
Locks - I did not use locks on my pack, but when I am travelling on public transportation, I frequently use zip strips (narrow, plastic strips that are used to bundle cable - cable ties). You can discourage people from opening the zippers on luggage by zip-locking them. Of course it would be easy to cut them off, or simply slash the bag, but it does slow down the casual thief or your cabinmate. They prove useful for a variety of other funtions too.
Once I got on the Camino, all the conversations and concerns about what to pack, etc, simply fell away. The truth is, you take what you take and as the experience unfolds, you learn a lot about who you are and what you really need (or want). I was grateful to discover that I could happily function with soooooo little.
Life is good...
"Ginn"
In Suny Santa Fe