1. OP isn't as bad a designer as he thinks. The goal isn't a beautiful page so much as "no trust destroying indicators" like weird fonts, bad formatting etc. Good Job!
2. OP could probably double their sales by offering the first chapter for free with email signup on the landing page and then offering a time-limited discount as part of an email sequence.
3. We (in the dev space) have a weird view of these sorts of things as we make relatively so much in salary. But $400/mo is a decent car payment, another way to thinking about this is that OP wrote himself a free car.
4. OP could have taken all this knowledge applied it in his job, etc. but by making this very public and consumable objective proof of his knowledge he's put a solid milestone in his career path. I've friends that have done similar and then picked up six figure consulting gigs, lucrative job offers, etc.
All around, I just want to say nicely done and that I would love to see more developers do similar.