By making marginal notes on a book, we mean "write between the lines" in the book when we read. Many people know that we have to "read between the lines" to get the most out of anything. In my opinion, "write between the lines" are even more important in the course of reading, and we are not likely to do the most efficient kindof reading unless we make marginal notes on the book.
Making marginal notes on the book is indispensable for the following reasons. First, it keeps us wide awake. If reading is accomplishing anything more than passing time, it must be active. We can't just let our eyes glide across the lines of a book and come up with an understanding of it. In the second place, reading, if it is active, is thinking, and thinking tends to express itself in words, spoken or written. The marked book is usually the thought through book. Finally, writing helps us remember the thought we had, or the thought the author expressed. The physical act of writing, with our own hand, brings words and sentences more sharply before our mind and preserves them better in our memory.
So if you have the habit of making marginal notes on the book when you read, keep at it. If you don't have the habit, start right now.