I feel like reading some fiction before getting into an instructional non-fiction book makes reading it a lot more fluid like when you warm up before doing an activity like lifting weights or doing a sport. For me, it feels like reading fiction is less taxing because contextualizing a story is less intellectually demanding than contextualizing real-world information. And I feel that your brain not having to be as analytical when reading fiction is what helps you relax into reading and that that relaxed nuerological state of assessing what you read becomes a buffer for when your brain gets too analytical when reading and overthinking non-fiction. Am I hitting on something or am I just unique?