Interestingly, no one so far has asked me why I am putting my book online for free. Rather, they’ve been asking how I got my publisher to agree to it. Assumptions have really changed!
The short answer to how is that I chose Cambridge University Press in part because I saw they had allowed Chris, Prabhakar, and Hinrich to put their book Introduction to Information Retrieval freely online before publishing it in dead tree form.
CUP hasn’t done this with all that many books, but so far it seems to lead to more sales. So let’s hope that happens with Search User Interfaces as well.
The second reason I chose CUP is they have published the bulk of the books related to human information seeking, including Gary Marchionini’s Information Seeking in Electronic Environments and Rik Belew’s Finding Out About. (BTW, I just noticed you can search these books and see hits in context at the CUP website!)
As for why, I’ve long been of the mindset that my writing has value so far as it has impact, and the best way to do that is to maximize how many people read it. Along with many others, I’ve long been an advocate of freeing the content of academic journals, since the knowledge produced by research should be dispersed as widely as possible, and in my field at least the academics to all the writing, reviewing, and editing. Not to mention that the government pays for a large chunk of the research being reported. People used to laugh at me when I started saying this, but thanks to the hard work of lots of other people, that ship in the slow process of turning.
But I suspect my primary reason for putting the book up free is laziness: now when I’m having a conversation about a search interfaces topic I can just say “if you want to know more, go to section S.X of my book.” It’s like a mental subroutine call.