KiwiNoma's advice is useful and there are threads on the site to give many practical hints about picking up a bit of Spanish. I have done three Caminoes and, while my Spanish remains basic and abominable, I discovered that making the effort made me many friends. My experience is that the Spanish love their language and happily will spend hours walking with you, speaking at the most basic level to encourage you to express yourself.
Of course, as you are on pilgrimage, I should not refer to the extraordinary incentive of making oneself understood to the many delightful and agreeable Spanish pilgrims one meets....
If you must read (and I must admit that I find it relaxing over a glass of the local tinto) the old Oxford pocket classics are lightweight, printed on bible paper-- they have a good selection of Victorian novels (Trollope, etc, wrote each chapter for weekly papers and, like TV series, one can read an episode at a time without losing track) and poetry. You really only need one-- 1,200 km from Montserrat to Santiago this year, and I barely finished The Last Chronicles of Barset.