Yes, reading too much is harmful to eyes.
Generally speaking, reading needs the concentration of our eyes, which means our eyes have to work with full attention. If you read too much, you eyes will feel tired and dry, and you want to close your eyes. In the long run, this leads to eye diseases and nearsightedness. So one should have a break now and then while reading. Doing eye exercise or looking at the landscape far away is helpful in reducing tiredness of eyes.
The light is very important when reading. The light shouldn't be too strong, shiny or dim. Reading in dim light is harmful to sight, one would suffer from nearsightedness if he reads too much or read in dim light or shiny light.
Long time eye use will cause vision problems such as myopia, glaucoma, cataract etc., especially in a dark ambient of environment. The vision will be affected at the first, and then the conjunctivitis even keratitis will appear after. Have you seen the blood shot on the eyes from some people who stay up late usually? That symptom is one of the representations of the inflammation symptom of bulbar conjunctiva. With 80%'s information is absorbed by your windows of your heart, you would better cherish this subtle organ. Once they were damaged, they are hardly to recovery with abundant of time and money. To relax your eyes at regular time. Make sure relax 10-15min after every hour's book reading. Looking far distance view, taking eye exercises or massage both ok. Hope you keep health with the eyes!
Just like drunk, reading too much makes nerves feel fatigue. If the brain slow down, not only eyes can be affected, but also other parts of body. We often feel our brains cannot think any more after a long time working and reading. That's because brains need time to take oxygen and blood supplements. Then let brain have a good rest, or double vision, dizzy and headache begin to occur. If you always reading too much, you'll read more near and near. Then myopia follows you. When nerves that govern eye movements being hurt, eye strain and dry eyes occur naturally. However, these eye problems are the basic problems of other eye disorders, such as glaucoma, conjunctivitis and red eyes. So you must have a limitation of reading if you want to have healthy eyes. Plus, when you feel tired after reading, please close your eyes and have a good rest. You also can drink a cup of black tea which is good to recover eye sight, even you can apply cucumber slices to keep eyes fresh.