Does reading improve mental health? There's evidence that it does.
It's long been known that reading improves overall cognitive ability. Studies have shown that there's a correlation between early literacy in children and higher IQ scores later in life, and that reading in older adulthood delays cognitive decline to some extent. Reading involves the decoding of symbols (letters) and forces the brain to interpret words in context--perhaps the brain requires this exercise to stay fit. Pictures don't have the same effect, because understanding them doesn't use the same brain functions.
But what about emotional health? There are several ways reading may improve emotional well-being. This rather long article in The New Yorker http://www.newyorker.com/culture/cultural-comment/can-reading-make-you-happier
on bibliotherapy cites some studies indicating that reading fiction improves the capacity for empathy and also, can be a "mindfulness" activity (see my previous post on mindfulness).
This brings up the issue of what type of reading we're talking about: Any type of reading may increase general cognitive function, but reading fiction, particularly the best fiction, may have special benefits: Good fiction convinces the reader, temporarily, of the "realness" of the characters, setting and events (thus inducing a trance state that removes the reader from his/her real-life troubles). Good fiction also includes well-developed, multi-dimensional characters and immerses the reader in the character or characters' inner and outer worlds, helping the reader see through their eyes. It's easy to see how this could increase the capacity for empathy.
The best writers portray villains who perhaps weren't always evil, and heroes who are flawed. These nuanced explorations of human nature may especially help readers with personality disorders. Persons with Borderline Personality Disorder in particular typically divide the world into polar opposites: Good v. Bad people, and simplistic or extreme interpretations of events. These tendencies are found in people with Borderline Personality Disorder even when the person has a high IQ, because these distortions are caused by dysfunctional ego defenses, not by low intelligence (see my previous post on Borderline Personality Disorder from Oct. 14, 2015 ).
Reading fiction might also help persons with depression: The characters in the best fiction grapple with conflicts and make mistakes, even when they are the novel's "heroes." Reading about their struggles might help self-critical persons moderate their self-criticism.
In short, reading is important, but it matters what you read. Many of the opinion essays, Facebook posts etc. that I come across on the internet are nothing but recitations of popular slogans and catchphrases. They often demonize opponents by using hyperbolic language. They often promote a code of "correct" language that is remarkably reminiscent of the "newspeak" of Orwell's classic dystopian speculative fiction novel "1984." The purpose seems to be to increase fear and hatred of opponents, as well groupthink and submission to self-promoted "authorities." These are the tools of authoritarianism and totalitarianism. This type of reading can have a negative effect on mental health by increasing anger. It might even negatively impact overall cognitive functioning by encouraging simplification.
I've noticed over the years that although reading complex novels doesn't prevent mental illness, my patients who read fiction often seem to cope better with their disorders, including serious disorders.
To increase general knowledge, I recommend reading reputable non-fiction books written by academic experts or by reputable journalists who are basing their works on facts and research, not on opinions. To increase emotional well-being, I recommend reading fiction (in any genre) that includes multi-dimensional characters. Maybe a book club is a therapy group!