I don't know whether you made the same observation but I found out that "book-thinking" (thoughts of protagonists in books) is very different from "authentic thinking". (I know the written word can only do so much, this is just the "basis" for my observation). Then I thought about how I think and I realised that I mostly think in pictures. There are no sentences or phrases in my head like "I need to go shopping", it's rather a picture of the aisle in the pupermarket that pops up in my head.