I learned about the concept of a Commonplace Book from Charlotte Mason studies. Once I figured out what one was, I realized I had been keeping one for many years. I have somewhat combined the Commonplace book and a prayer journal. I have noticed that as I pray, God continually answers me in everything I read. I can go back and see what I have been talking about with God and then how He answered me through poetry, literature, and the words of others. I love being able to reread the things that have touched me. I have memorized many lines of a poem and verses of scripture by employing this method. I put milestones about my children and a log of things going on in our lives. Looking back, I can glorify God seeing His hand over time. It's beautiful to recall.
You are wondering what on earth a common place book is. Sir Francis Bacon, John Milton, Samuel Coleridge, and Mark Twain kept them. These great writers attended no formal courses in how to write. They simply copied beautiful portions of great works that spoke to them. Often they would commit them to memory. A Commonplace book is a book of collected quotes, poems, songs, prayers, recipes, thoughts on art, observations of nature and the like. You can add drawings of things you observe. I copy many scriptures and poems about God into my book. This leads me to do as Philippians 4:8 says and to think on pure and noble things.
Several great writers have even had their Commonplace books published.