Hey, museum worker and professional historian here. I have led a very successful history book discussion group for almost ten years. I just want to say that it's ok to not retain everything. Seriously. There's no award at the end of life for remembering everything you read. It's sufficient that you get a "takeaway idea" from each book or maybe you just know that you found something interesting along the way. Don't stress!

If you want, maybe just keep a journal or spreadsheet with the basics of the book. If you ever think "hey, what did that author say about this topic that just came up in my life," then look the book up again.

The book discussion group also helps retain information and digest it with multiple perspectives. Check out your local library and other organizations.