Hey,
When we read in low light,it leads to eye strain and over time our eyes may become weak leading to decline in the eyesight. That is why it is always advisable to have sufficient light while reading. Also,the shadow of hands will not really affect your vision as long as you are comfortable and getting enough light. However,You should try to sit in a right posture and maintain sufficient distance from your books while reading. Apart from that,it is always good to see an oculist if you feel discomfort in eyes.
Hope this helps.
Good luck.