Hey, capsule_toy is totally right — that error shows up if you use an email associated with an existing account but claim that you're a new user. Shoot me an email and I'd be happy to give you a copy of the book for free!

Happened to me. Turned out I already had an account, but forgot about it. If you try to register an account w/ an existing e-mail, it'll tell you if it's already being used. The checkout process after I was logged in went fine.

Also, I tried the password reset before realizing I had it stored in 1Pass, and the reset didn't work. Site could use some work on functionality.