Strain but no harm. Contrary to popular belief, reading in dim light does not do any long-term damage to the eye itself or weaken the eyesight. For the short-term, however, it can cause some eyestrain, which is just fatigue in the muscles of the eyes. If your eyes feel sore or your vision is less crisp than usual, it is time to close your eyes and rest them (and maybe dream about that great novel you're reading!).
No. Reading in low light may strain your eyes and cause dryness and discomfort, but no lasting damage. Always read with proper lighting. Good luck.