It has continued to sell well, and about two years ago I "earned out," meaning my royalties exceeded my advance, so now every six months I get a royalty check.
I've also agreed to several deals that allow my book to be translated and sold in different countries. Those deals are usually on the small side, but the only work involved is picking up a pen and signing the paperwork.
The success of that first book has allowed me to write several related books, and while the advances I've received for those don't count as passive income, my hope is that they will generate some passive income in the future, if I should be so fortunate as to earn out again.
To anyone who is thinking about writing a book, I offer this advice:
There are some cases where it makes sense to self-publish even if you can get a book deal. Niche e-books on technical topics written by people with large built-in audiences are one example.
But in general, if you can get an agent, get an agent, and if you can get a book deal, get a book deal.