The ‘why’ game is astounding in that it quickly highlights the things that you do and don’t know about everyday systems. I’ve just tried it with myself and it quickly revealed truths that I thought were solidified in understanding but that were in fact, merely analogies and ‘hand me down’ understanding.
Writing a review of a book helps in remembering its content, it’s mood and your attitudes and reactions to it. Speaking about it aloud or sharing the main ideas with a friend can also help in consolidating that memory.
I find myself constantly forgetting key details from books, including names of main characters and places; however can normally hold on to my feelings and ideas of the purpose of the novel and the main attitude that it has driven into my understanding of it.