I followed the steps exactly as in the readme:
$ cargo install mdbook
Updating registry `https://github.com/rust-lang/crates.io-index`
error: binary `mdbook` already exists in destination as part of `mdbook v0.0.14`
Add --force to overwrite
$ git clone https://github.com/rust-lang/book.git rust-book
Cloning into 'rust-book'...
remote: Counting objects: 10551, done.
remote: Compressing objects: 100% (173/173), done.
remote: Total 10551 (delta 99), reused 0 (delta 0), pack-reused 10374
Receiving objects: 100% (10551/10551), 12.48 MiB | 1.35 MiB/s, done.
Resolving deltas: 100% (8040/8040), done.
$ cd rust-book
➜ rust-book git:(master) mdbook build
➜ rust-book git:(master) ✗ open -a "Firefox" book/index.html
What I see in firefox is an empty book:

Any ideas?
Stuff that might be relevant:
Sorry, the README is slightly out of date, we did some reorganizing.
You need to run this in the first-edition and second-edition directories, depending on which book you want to build.
I'll be fixing this tomorrow; I'm going to leave this open to track it. So sorry about that!
Went ahead and fixed this. Also wanted to mention that I love your avatar, @jesperp! I made that same broken image image my avatar at a previous job, and accidentally made a coworker think his gravatar integration was broken!! 🤣