Keeping notes you will actually reread
A long journal is hard to keep. A short note is easy. One line per day, with the date and the chapter, is enough to show you how your reading has changed over months.
Write what surprised you
Write the thing that surprised you, not a summary. Surprise is what you are most likely to remember and to return to.
Keep it in one place
Use one notebook or one note on your phone. Notes spread across several places are the ones that get lost.