Memory alone will fail you.
In 2017, I read 89 books, 75+ of which were nonfiction. I applied ideas, changed habits, and learned much.
But 3 years later, I can only remember the key ideas of a handful of books. The knowledge is still in my brain, but an idea only returns as an eureka. It's hard to build enduring habits on fleeting memories.
I started my book notes to address this problem. Now, I'll still remember the key ideas from reading a book, and I can record other interesting ideas for my future self.
Save ideas that you find interesting. Use whatever you have: paper and pen, a text file, the back of a receipt.