I get a little soreness too. I think it's from constant motion of turning pages...the arm never gets a chance to rest! My hand falls asleep sometimes too. I found that better posture fixes that.
This is kind of silly in a way 
My great grandfather would slap me around if he heard me complaining my arm hurt from reading! Come to think of it, I'm a little sore from moving that 500 Megabyte file to my desktop this morning
