$15k certainly sounds impressive, but how many hours have been spent researching, writing and marketing the book? Only once we have a ratio'd figure to compare can we claim success.
1) You build a mailing list of people who have bought from you in the past. HUGE value here.
2) You get some extra personal brand exposure. Invitations to speak at conferences, podcast interviews, and so on.
3) Passive income is the gift that keeps on giving.
But it certainly increased his percieved value.
It's SO not just $15k.
Think about all that crippling anxiety over building something no one wants. Or the psychological wounds still bleeding raw from a product attempt that failed for whatever reason.
One small step in the bank statement, one giant leap toward 37signals.
My book, however, people want. I looked for what developers need first and then built a product around that, rather than thinking of something that might be nice and try to get people to buy it (like my CMS hosting).
There may also be some non-financial benefits of going through the experience as others have commented, plus the sense of accomplishment.
It does, however, come from real-world work. My client projects have been successful from techniques in the book. I'd need to split hairs to determine what time was spent where and in the end I just needed to write.
It's the same problem with choosing a ebook platform. No tool does you any good until you are actually using it. I'm currently writing in Apple's Pages app because it was the nearest and easiest way to just get started.
The benefit of a product is that it can become many things and the hours put into it can be easily won back with more sales.