Beware, there is a sign-in wall on this "free" introduction that comes up after a couple of topics.
Edit: this seems to be the case only on the mobile version
The intent behind the sign-in seems to be to allow you to participate in the built-in SRS system for learning the material. Obviously there's no guarantee that behind the scenes your email is not being sold off to NefariousMarketers Inc., but given the past projects and work of both authors, that seems perhaps unlikely.