Mar and Rain (2015) reported on four different studies in which university students were asked how much fiction and nonfiction they read and also took an author recognition test for fiction and nonfiction reading material. Results were similar for both measures: The amount of fiction read was a consistent (but modest) predictor of various measures of verbal ability, including a test of synonyms (vocabulary) and reading comprehension.
The amount of nonfiction read was a consistently weaker predictor of verbal ability.
Mar, R. and Rain, M. 2015. Narrative fiction and expository nonfiction differentially predict verbal ability. Scientific Studies of Reading 19: 419-433
Krashen, S. 2015. The great fiction/nonfiction Debate (with the title Fact or Fiction: The Plot Thickens). Language Magazine, November.