Today was the first surprise Gumroad notification of a sale of the book to a customer I hadn't spoken to before!
My book had kind of been languishing, mostly due to lack of marketing effort on my part. To get a sale now means that someone found some previous material of mine (maybe a post here?) and purchased.
It's heartening to see that my past material worked, and a reminder that the 'stack the bricks' strategy could indeed work.
On a personal note, it's also a welcome development since my day job and personal life are both very busy right now - so it's great to see past seeds planted now bear fruit.