There's also a great Adam Neely video about it (his thesis is that TRB is a "jazz shibboleth").
Thanks, I had this in a lower comment but it should really be at the top.
Previous discussion: https://news.ycombinator.com/item?id=26737142
https://99percentinvisible.org/episode/the-real-book/
Link has a lot of photos of a "real" Real Book.