37signals is a web 2.0 company behind several successful web applications. They’re also professional, successful, published authors. About a month ago they self-published a PDF-only book about their software philosophy, Getting Real, which they sold only online for $19 (you could also buy a 10-copy license for $50). In 30 days they’ve sold 5,750 copies and raked in $120,000.
If this is your test case for online publishing, it’s the best possible scenario. Getting Real is very well written, edited and layed out. It’s no amateur job. 37signals also has a built in marketing base of eager buyers. So publishers don’t need to be shaking in their boots just yet. But they better be paying attention.