I have the same problem, I've lost a lot of weight due to not eating (stress) and my back was screwed royally due to sitting all day and sleeping on an air mattress for three months. When times were tough (which was all the time) I had random body aches and pains.
Reading one book is like doing a workout of a single exercise (e.g. Grace – 30 reps, Clean & Jerk @135lbs), so when you have 5-6 hours to dedicate to reading, switching between different books every hour or so will help keep you moving.