My Early Modern Literature and Culture course (aka Shakespeare's Sister) at Marylhurst University introduced me to commonplace books this term. These books have been around for centuries, and they may be a great choice for you to record important passages from books, personal reflections or observations, or the occasional sketch or two. Thinking back to diaries of family members that I remember coming across as a child, I suspect I've seen these before without recognizing them for what they were.
I love the idea of the commonplace book, and I will certainly recommend the practice. In fact, people often ask me how to become a better writer, and this may become one of the tools I talk about in the future--e.g. reading, writing, and living. For years, I did used to carry a writer's notebook. I remember taking mental notes of characters or dialogue from public places, and these daily snapshots would usually find themselves added to my trusty notebook. (When I considered a law enforcement career in the late 1980s and early 1990s, pocket notebooks took on a whole different kind of purpose in recording notes on techniques and...people.) Today, I tend to rely on my camera to remind me of important scenes or landscapes. Conveying a solid and sharp sense of place within my fiction is tremendously important to me, and this method tends to work well. When it comes to recording dialogue, I usually record notes on electronic devices, but I do occasionally use old fashioned notebooks or journals. The digital audio recorder can also be useful.
For taking notes of passages of writing that are personally important, there are many different approaches I use on a daily basis. Much to my mother's disappointment, I'm afraid, I tend to highlight important passages in traditional books. I also enjoy using the "highlighting" feature found in eBooks. For a writer, even published articles can be a way to remember favorite quotes, as this article is a single example. Applications such as Evernote are also a great way to record important information along these lines. (You can even record You Tube videos!) In short, then, I probably won't be changing how I take notes insofar as this experience is concerned, but that doesn't mean it wasn't a valuable experience. I would add that the commonplace exercise is helpful in enhancing attention to detail within works. For me, it also offers an interesting insight into where my own note-taking methods could be improved and strengthened.
So, when it comes to commonplace books, these are great tools for the present, and they have the potential of providing glimpses and insights of the "hidden transcripts" of those who lived before us. Are they something I will personally continue? Probably not, but the fault lies with me alone. You see, I really can't stand my own handwriting. If this doesn't present a challenge for you personally, go for it!
(Here are some additional examples of commonplace books from Pinterest.)