[gamble]

I'm pretty sure that 90% of the books published by O'Reilly and the like are written by consultants as self-promotion, with no expectation that they'll provide a return on their own. Presumably O'Reilly has adapted to take advantage of this fact.

[unreal37]

It's also possible that writers start out really excited to make a book to help people, and then in the end all they have left is the self-promotion out of it once their publisher didn't do anything to promote it.