Yes, that's true. One of my best contracts was obtained because of the book. It is surprising how much caché a published book has. And for folks looking to make a living from Open Source software, I'd recommend writing a book about your project, since it gives you a huge amount of credibility within the community.

Though there are some publishers who put out crappy books that I would think would have a negative impact on your credibility. I won't name names, but two folks I know to be very smart and definitely experts in their field (I believe both are users here, though not frequent users), wrote books for Apress and Packt, respectively...and both are pretty sloppy, occasionally repetitive, and sometimes even wrong. The funny thing is that I know they're both capable of writing great documentation, because they've done it for their projects for years.

I had quite a bit of editor input throughout the process of writing my book for No Starch, and I was one of the technical readers for an O'Reilly title, so I know that a lot of effort goes into making sure books from top name publishers (and mid-tiers like No Starch) are readable and reasonably accurate.

Then again, I don't know that anyone who ever hired me because of my book actually read it. So maybe it doesn't matter if the book is sloppy. Maybe it just matters that it's available at Amazon.