I got a raise of around $30,000 as a result (ish, hard to measure) of writing my technical book. I'm also sufficiently well-known that I'm unlikely to ever have trouble getting a Ruby job. That's hard to quantify exactly, but I think it's fair to say it's worth $5,000-$10,000 annually, as a rough equivalent.
(My book sells for around $30-$40 rather than $15, but the math is similar.)
(My book sells for around $30-$40 rather than $15, but the math is similar.)