It’s an interesting dead metaphor: light reading. I guess it’s intended to convey the same idea as ‘easy listening’: not challenging, not disturbing. It’s not about shining light on anything; it’s just not ‘heavy’. If you do too much of it, your mental and emotional muscles will atrophy. But then who wants always to be straining?
My personal preference is for light reading that still has a little heft to it. A lot of good genre fiction is like that. Good science fiction packs in ideas with its page-turning narrative. So do spy novels, such as those by John le Carré. Good crime novels, a very popular genre, often include wonderfully complex and interesting characters and intricate plots. And I should add that often a large part of the appeal is the quality of the prose. I’m a fan of James Lee Burke, for his description of New Orleans and the Louisiana bayou country often rises to the status of poetry.
It’s worth remembering that a lot of books now considered heavy, and that students consider a burden to lift, were once the popular, ‘light’ reading of large audiences. Both Dickens and Dostoevsky, for example, were both extremely popular novelists in their time, and both sometimes wrote what could be considered ‘crime’ novels. It seems fitting that it was a poet, Edgar Allan Poe, who actually invented the genre with his tale, “The Purloined Letter”.