Here are some ideas on what do journal about with each book!
Character list and analysis
Your rating and average ratings
Settings (I keep a map and color them in where each book was set)
Comparision to an adaption (TV/Movie)
Steam rating (romance)
Who you would like see play the characters in an adaption
Dates when you started and finished the book
Tracking the time that you read
Why did you read this book?
Any takeaways from the book?
Journal each chapter (helpful is discussing the book)
New words and definitions
Mood of the book
Monthly wrap up
Yearly favorite book bracket (like March madness)
Anticipated new books
DNF list and why
Track books that you bought
Track books that you lent out to others
Track due dates for library books
Artwork for the book
Favorites: character, scene, quotes, etc.
Would you recommend the book?
Sometimes you can find discussion questions or you can make up your own questions in case you were to ever discuss the book
Notes as you read
I use a combination of all the above when I journal. It helps with my reviews too. You can make them yourself, buy them with prompts to fill in, or buy printable pages. You can do a digital, paper, or even an app to journal. I prefer some printable pages and some I like to do myself. I also keep a few Excel spreadsheets and use online trackers like Goodreads and StoryGraph.