I've been asked to write a book for a few years by potential readers, and a few times by potential publishers, but I was very disinterested in doing so. For most people capable of writing tech books, writing books is a very poor use of time under any economic calculus. The typical model is that you and the publisher agree to do the book, then you're paid an advance (sometimes in installments contingent on milestones). You earn a royalty on each sale of the book, with the royalties first going to pay back ("earn out") the advance, and then being paid to you quarterly.
This sounds reasonable, until you hear the numbers (typical advance for a tech book from a first time author: $5k, typical royalty percentage: 8% on a physical copy, 15% on an ebook). They're not universally that bad -- an author or two from Apress and Pragmatic Programmer wrote me to say they were genuinely happy with their experiences -- but that's sort of the general flavor of things.
If you do the math, it is highly unlikely your book will recoup the advance, and that is indeed what happens to most tech books. So you've traded months of full-time labor writing and rewriting, plus additional months of part-time work hawking the book to anyone who will listen, in return for $5k. That is not a rational use of time for anybody who can e.g. do contract programming, which (one would assume) virtually all tech book writers could do.
When I said as much to publishers who asked me to write things, they said "Ahh, but the book will allow you to establish your bona fides", and I do not believe that making stupid decisions for my business successfully demonstrates to clients that I will make smart decisions for theirs. To my view of things, I'd rather have lots of folks be able to read my writing than have it blessed by a gatekeeper but widely unavailable.
But blog posts do leave a lot to be desired for teaching people. For one, my blog is very disorganized, since I have six years of back-issues which appear in order of "what I was working on that week", and they're of varying quality and relevance, particularly early in my career. Something with a bit of structure and curation seemed like it would add value. Plus, I am spelling- and design-challenged, so having an editor and someone with a sense of style would be a plus.
That's basically what Hyperink pitched: curating the best of the blog into a book, complete with editing and design help, with some extra essays added. It's far less work than scratch-writing a book, the terms are more equitable (no advance, equitable royalty split), and it provides incremental value to readers over the blog.
I don't expect to make a huge amount of money on it, relative to the business in general, but the idea sounded attractive enough to try it as an experiment.