2. Underline essential quotes and ideas on each page

As I read, I ask myself the question: “If I was to go back and skim-read this book, what few sentences on this page would I need to read to understand the core concept or the essential elements of the plot?” The answer to that question determines what parts I underline.

Underlining or highlighting is not an exact science. Different ideas and quotes resonate with different people. Sometimes my wife reads a book that I’ve read and marked up, and she’s surprised by the types of things I underlined. They don’t make sense to her, and they’re not really supposed to. Your notes are for you — not for anyone else.

While I read, I also take note of new words I don’t know. For example, the picture above is from the book Stein on Writing by Sol Stein. Page 116 of that book contained the word “antediluvian,” which I had never heard before, so I looked it up and jotted the definition on the bottom of the page. I’ve been using a backward pilcrow (the paragraph symbol) to designate new vocab words, but you can use any symbol you’d like.

3. Record your biggest takeaways inside the book’s back cover

This is the most important part of my marginalia system. Years ago, I realized that if I wanted to thumb back through an old book to find a quote or concept, I had to scan through every page to find what I wanted.

That seemed ridiculous, and I knew there must be a better way to summarize what I had learned and where I could find it in the book, so I began using the inside back cover of every book to record my primary takeaways.

Here’s an example from Walter Isaacson’s biography Benjamin Franklin:

If you look closely, you’ll notice several different types of notes in the image above. Here’s a description of those notes from left to right:

• Favorite Quotes — The upper left corner includes page numbers of a few of the author’s best quotes.
• Key Takeaways — The main section on the left page includes a page number alongside each core insight from the book. I usually choose about 15–25 main takeaways for every book.
• Date Finished — The bottom left corner shows the date I finished reading the book. If and when I re-read this book, I’ll write the re-read date as well.
• Book Recommendations — Most authors are insatiable readers. Many writers consume 10–50 books while researching a book, and they generally reference those books throughout the text, along with sprinkling in some of their other favorite books. I write the author’s recommendations in the upper right corner of the back cover. If I want to dive deeper into some topic from the book, those recommendations give me a great starting point.
• Personality/Character Traits — In the bottom right corner, I jotted down a few things I learned about Benjamin Franklin as a person: what made him successful, what he struggled with, etc. I’ve begun to write lists like this in biographies, and it’s fascinating to see the common character traits between successful people in diverse fields (curiosity, work ethic, etc.).

The types of things I write in the back cover of a book differ based upon the type of book I’m reading. For example, the last bullet point above about Personality/Character Traits wouldn’t make sense in a book about economics, psychology, or leadership. For those types of books, I jot down other notes, like what I plan to apply from that book to my life.

I also write notes in fictional books. I’ve found that the notes help me follow along with the plot and remember what happened in the story. For fiction, I usually write a character list on one of the pages of the book’s back cover.

Depending upon the type of book, I’ve also begun to write timelines in the back of some books. I primarily do this in history books, biographies, memoirs, and even some business books. Here’s a picture of my timeline from the Benjamin Franklin biography: