If the books are poly-related and you're not trying to push them with every posting, I don't see any problems with it.
Our spam policy is to keep the place friendly and the focus on poly living. Spam in sig files tied to anything not poly is out because we're not here to help folks push their wares just because the folks happen to also be poly. This is not a marketplace, it's really a decommodified zone.
We allow links to other poly sites, even if the site is for an author pushing a book on poly, as a service to members who may want more information. Were one of the authors to show up here and provide a link in a sig file...I think we'd have to discuss that and decide how to handle it then.
This is as good a board as any to post questions. I'd recommend posting them in the Guidelines sticky discussion, though, as posts there get my attention more quickly (and, I suspect, the same applies for River and Olivier) and the answers are likely pertinent to further expansion of the guidelines and such.