your navel, an inner being, the meaning of life, the meaning of death and what follows, there are a myriad of topics.
Might I suggest you consider this:
Find a VERY quiet place away from the crowds...close your eyes and breathe. What do you hear and what does it signify. Do you hear birds, crickets, cars off in the distance, or a jet flying overhead? What does that mean to you, to the world, to the Camino.
Find a fairly noisy spot: Seek out a sound different from the normal press of sounds: a baby crying in the distance, a train whistle, bells on a cart. Why did that particular sound appeal to you among all the others. Could it be you have a new baby in the family you miss, did you arrive for you Camino by train and it's calling to you...helping you along, do the bells...as small as they are still ring true amid the clatter and the great bells of the cathedral?
Why...seek out! Well, to be practical...you don't have to carry the extra weight of a book. You only have to engage your mind!
Might I suggest a journal to write rather than to read? You can also read your journal on the Way... how did feel, what did I see, what were my reflections a week ago, two weeks ago? Additionally, there are many pamphlets along the Way: prayers, writings, reflections...I collected many to my detriment...they ended up weighing down my pack, but I wouldnt' part with them and am now glad to have them! It is truly amazing what paper weighs!!!
I took a journal and tried to write in it every night depending on how tired I was. I can even tell that by my handwriting. I have enjoyed reading it so much since I have been home. It reminds me of so many little things that I would have forgotten and it is nice to look back to see exactly where I was each day.