I wrote a book[1] a year and a half ago that just recently crossed $42k in total revenue. These days it consistently earns $1.5k/mo without much further input from me, other than tweaks to the landing page copy and updates once in awhile when Stripe or Rails changes significantly.
Why not both? Make a custom domain for the book, then treat your own site as if you were guest-blogging on it to pump link juice to the custom domain.
That way, your venture ends up with the combined page-rank of your personal brand and the exact-domain bonus. (Whereas if you just redirect from one to the other, you lose one or the other.)
If you have a domain name already, use it. I launched with just a page on my blog and later moved it over to a separate domain. I don't think it matters in the long run, and in the short run you get a boost from your personal domain that people already know.
I use Stripe (and sometimes PayPal) so I get 97% of the gross revenue. Monthly recurring expenses are a cheap VPS to run the app and Facebook retargeting fees.
[1]: https://www.masteringmodernpayments.com
Edit: If you'd like to read a preview, you can do so here: https://www.masteringmodernpayments.com/read