What does this aphorism mean "Read much, but not too many books."?
Read and understand what's going on around you; news articles, letters, etc.
My take on Benjamin Franklin's quote, "Read much, but not too many books," is that instead of automatically thinking to a book when referring to reading, take a look at news articles and letters which will give you insight on what's happening around you. For example, reading an essay on human rights will give you a better idea of the world than reading a fiction book will.