Lately, I've been using GitHub Wiki pages to keep notes on random topics I find interesting. Books I read, articles that I want to refer back to, a weird mathematical equation that helped me solve a problem, etc...
It depends on the book, and subject matter. I might turn notes on The DevOps Handbook into more of a technical format but a book like The Lean Startup might have just Chapter titles with bullet points. I don’t think I’d use it to review books though.