Troy Tao

There is a link showing how to read specific version of a [gitbook](https://help.gitbook.com/content/how-can-i-access-a-specific-version.html), and it depends on branch, tag, or SHA of a commit of the repo. So I made...

@bennett000 I rebased https://github.com/rangle/ngCourse2/pull/756, and opened the E-Books downloads setting of the demo book. But the provided download link does not seems to be working... Could you take a look...

@bennett000 It turns out that to generate pdf locally using `gitbook pdf` command, we need to download [Calibre](https://calibre-ebook.com/). And by testing the pdf book locally, I found one issue about...