If you are like a lot of others, you may make notes about the research you do before making your book, or keywords or ideas for the book and you write them down, put on post it notes, or even in notepad, word, etc.
You can now keep notes all within your project if you want.
Down on your project information bar on the bottom left of the work area, just click on the icon on the left.
and when you do a text box will appear where you can write your notes.
Just write some notes and press "Save". Next time you come back, you will see your previous notes and add more, edit, or delete the old ones.