Your question includes a solution too, and that is the wrong solution.
Using the phone's light as a reading light is definitely an unfortunate idea in the long run, because it will enforce faster discharge of the battery, followed by other charges. That is exactly what kills phone batteries.
It is a lot more recommended to buy some cheap appliance, designed to do exactly that task. These appliances can be head (your own :)) mounted, wall mounted, free standing on a table, even attachable with a (big) clip to the book. They might provide both battery-stored energy, as well as connection to the wall socket (possibly through an adapter).
Just throw a search on your preferred search engine, and choose from the large variety something that suits your needs best.