A 2006 journal from Facebook CEO Mark Zuckerberg had lots of ideas about potential concepts for the company, including one that would let users create profiles for people who weren’t actually on the social network yet.

Details about the journal come from Wired editor at large Steven Levy, who is publishing a book about the history of Facebook and published an excerpt of that book in Wired today. The full excerpt has all sorts of juicy details about Facebook and Zuckerberg.

Much of the excerpt focuses on Zuckerberg’s studious journaling and how he used it as a way to muse to himself about early ideas for Facebook:

[Zuckerberg] sketched out product ideas, diagrammed coding approaches, and slipped in bits of his philosophy. Page after page were filled with straight lines of text, bullet-pointed feature lists, flow charts.

Zuckerberg would sometimes even drop photocopied pages from his journal onto Facebook employees’ desks as a way to share ideas or changes to be made to Facebook.

Levy says he got his hands on 17 pages of one of those journals, which dated back to May 2006 and which Zuckerberg had purportedly named the “Book of Change.” In it, Zuckerberg apparently wrote about opening Facebook up to everyone (at the time, Facebook was still exclusive to college and high school networks), early thoughts and ideas for News Feed, and one particularly disconcerting idea he called “Dark Profiles:”

These would be Facebook pages for people who, whether by omission or intention, had not signed up for Facebook. The idea was to allow users to create these profiles for their friends—or really just about anyone who didn’t have a Facebook account—with nothing more than a name and email address. Once the profile existed, anyone would be able to add information to it, like biographical details or interests.

According to Levy, some work was done on making hidden profiles, and Facebook has said that it doesn’t make profiles for people who aren’t on Facebook.

Zuckerberg has destroyed most of his notebooks for privacy reasons, reports Levy. That’s interesting in light of Zuckerberg himself retconning the founding story of Facebook last year — the journals could have provided more detail about the history of the company, if they still existed.

Levy’s excerpt also features a few other interesting tidbits, including one about the Silicon Valley legend around Zuckerberg’s choice to decline a $1 billion acquisition offer from Yahoo — apparently, Zuckerberg had actually verbally accepted the offer, but a negotiation choice by Yahoo let Zuckerberg end the acquisition talks with Yahoo:

He did verbally accept the offer, but then Yahoo CEO Terry Semel made a tactical error, asking to renegotiate terms because his company’s stock had taken a downturn. Zuckerberg used that as an opportunity to end the talks. He believed that the two products he wrote about in the Book of Change would make Facebook more valuable.

Facebook COO Sheryl Sandberg told Levy exactly what she had been hired for when Zuckerberg first brought her on:

Zuckerberg would be the lord of engineering—what Facebook built—and Sandberg would be in charge of everything Zuckerberg wasn’t interested in, including sales, policy, legal, content moderation, and, eventually, much of security. “It was very easy,” Sandberg told me. “He took product, and I took the rest.”

And apparently, after the consistent criticism of Facebook following the 2016 election, Zuckerberg told his executive team to consider him a wartime CEO that would have to be more decisive than in the past.

Levy’s full excerpt is long, but very interesting, and I highly recommend that you go read it.