Something to think about: Based on the answers here and in the related Reddit thread it seems that information products (e-books, targeted blog posts, link collections) still generate better revenue (either sales or ad revenue) than pure software products as side projects.
A lot more people who write books talk about their experience than those who run a niche profitable SaaS app for obvious reasons (competitive advantage).
In my experience, it's much easier to have a stable recurring revenue from a SaaS app than a book. Books go out of date much faster.
On the other hand, a book can't go bankrupt/get shut down/go out of service after you've bought it and invested time into. In that sense, it's a safer purchase.
