I think this kind of project is open to anyone who wants to do it.
The information that is available on the SMF Wiki is free under a share-alike license. So you can sell information from the wiki. As long as you also make that same information available for free. And make clear to prospective purchasers that said information is also available for free.
As a team member, I think you would have to run it past management whether it is OK for you to sell a support-related book. This is the kind of support the SMF team volunteers offer for free.
Why should you have to run that past management? How many people run books on Linux and other Open Source technologies past the management of those projects before publishing them? Unless they wanted us to officially endorse the book, there's no reason why we need to approve of the book's contents prior to its publication. Any issues that arose could easily be dealt with after the publication, and I don't think most of us have time to sit around and discuss whether or not we approve of what someone wants to publish.
Wiki content is covered by the Creative Commons Attribution-ShareAlike
license (which really needs to be displayed at the bottom of the page instead of being buried in the wiki's "About" page). Simply put, if anyone wants to use our wiki content, they have to give us credit for it and they have to distribute it under the same license or a similar one.
Theoretically, you can profit from distribution of a copy of the online manual (since that particular license does not prohibit using the work for commercial purposes), but I'm not sure why anyone would want to do that (and, if that's a problem, there's always the "Non-Commercial" version of that license, which prevents people from using the content for commercial purposes).