Reading, in itself, is a cognitive activity that involves the interpretation of script and its translation into concepts. So there is an inherent stimulation of the brain during the process of reading. Some recent research suggests that there is more mental activity involved in reading, than in the more passive observation of a television screen, because the absence of visual information when reading, requires a person to imagine the people and places from their description. This requires more mental effort and therefore, more brain stimulation.
Of course there is a significant difference between reading a book on quantum physics and reading a formula-written romance novel. The content of a book determines the degree of mental stimulation. A book with heavy mathematical formulas, graphs and complex ideas, is obviously a strong mental stimulus, while a romance novel or a celebrity magazine will not be cognitively demanding.
Great fiction can be extremely beneficial in increasing a person’s knowledge and hence, what is called their “crystallized intelligence.” The classical authors like Shakespeare, Shaw and Tolstoy teach us a great deal about human nature. Authors of historical novels can provide us with deep insights into present and past cultures, giving us a glimpse of how people lived in earlier times and in different cultural environments with diverse moral codes. Science fiction can stimulate us to think about issues like artificial intelligence and its implications. Dystopic novels like Orwell’s 1984 and Huxley’s Brave New World, provide insights into forms of government that could threaten human sovereignty. In short, reading of any kind is beneficial to cognitive growth, although the degree of mental stimulation will vary significantly with the content.
However, if fiction is read to the exclusion of non-fiction, the reader is unable to learn the many things that come from reading non-fiction. Today, knowledge is growing at an exponential rate, unprecedented in any previous era, and the need for reading non-fiction is greater than ever. Information in investments, economics, science and health is displayed in graphs, equations and statistics that must be interpreted and analyzed. So the short answer to the question is: read as much as you can and attempt to expand your reading across as many domains as possible for maximum brain stimulation.