From the rules: If one can put one's book in the bookstore and link to it there, and one can link to one's webpage which might include an ad for the book(s) and one can put the cover of one's book as one's avatar, why couldn't one put a link to one's book in one's sig? If you want them to link to it through the Bookstore, that makes sense. I think this needs a clarification.
Linking to the Bookstore is best, but right now it seems it's not possible to have a book specific URL in the bookstore You can only link to the bookstore in general and then you'll have to use the search function. I agree linking in sigs needs some clarification. We're discussing what criteria/rules should govern when an Amazon link to one's own work is allowed at the moment, so hopefully things will get cleared up soon!
Personally i dont see the harm in having an unobtrusive link to people's books in their signature. We're all hear to discuss writing and the ultimate goal of a lot of us is to see our work published. Seems silly to hide it away really.
Rules concerning links in this (and any) forum don't spring from keeping good, venue-oriented content out. Rules like this come from concerns for dealing with spam. I absolutely agree that the end result we're all hoping for, publication, is not a topic that should in any way be made taboo, be it in direct discussion of the paradigm or in display of the thing itself. The current staff has been instrumental in bringing in new areas to our forum for such discussion of these things that were once absent. Anyone who has ever been charged with the maintenance of a busy forum will know, first hand, just what a PITA spam is and just how conniving and cunning spammers are in disguising themselves and the Luis Vuitton Bags & Uggs & Get Ripped on Testosterone products they want to push on people. The plainly obvious Alphabet Soup spammers that one sees posting bizarrely broken love poems to the gods are the neophytes, the sophomores of the spam world. There are many other spammers who do a much better job at looking like real members. Any rules we have concerning the limiting of links, comes from guarding against that. This is why, as @Mckk mentioned, we are looking at how to best implement what is obviously a good idea without subjecting either the forum population to garbage spam or the staff to its cleanup.
I'll just go with the flow. Maybe it could be a privilege that's allowed for poeople who've been members for a certain amount of time and have made a certain amount of posts. I've edited accordingly anyway