So you're in the dean's office for some kind of review. Maybe you're up for tenure.
Dean: So what have you done over these past X years?
You: I wrote a paper that got accepted by Y. It will appear next year. Oh! And I wrote a book!
Dean: Excellent. Who's the publisher?
Dean: How many people are using it?
Dean: Has it been well-received? Do people like it? How good is it?
You: Errrr... Ummmm.... aaaaa....
Let's face it. The majority of academic authors are not publishing books for the retail sales profits. They do it out of love of subject and the hope that it'll help their academic careers. Open-source publishing gets your work out there, but once it's out there, you get little, if any, formal feedback (aside from a few emails from grateful users).
Editing, feedback, reviews, and formal assessment are areas that are nearly non-existent in open-source publishing. Without these mechanisms, dean's have no way to give you credit for your work, adopters have no way of comparing your work to similar commercial works, and you have no way of effectively and efficiently improving your book.
There is a lot of work to be done in strengthening these areas of open-source publishing. However, there are some tools available right now to help fill some of the gaps. In fact, an excellent tool is the blogging site you are reading right now.
Use one of the many free blogging sites out there to set up the public face of your open-source publication. Most of these sites offer commenting systems, lists of followers, RSS notification, etc. Once you establish your blog as the go-to place for information regarding your text, you give your users a place to offer feedback, show interest and get notified of changes. In addition, a busy blog site, gives you something to show people to convince them of the worth of your work.