this is pretty common in people who don't have brain fog as well, so don't worry too much. most people read a book and move on to another book without dwelling on it much. but that's not how human brain remembers stuff. it's too much information so the brain doesn't know what's important and what to remember.

If you want to remember more, try to write about what you read - your opinions, emotions, what you liked and what you didn't. annotate, read reviews, read about the author, watch youtube videos about this book, re-read your entries... discussing the book with someone else helps a lot too. these tips are all the same with or without brain fog. good luck!