I don't think that the issue of external links on Wikibooks or any other Wikimedia project really has to do much with wheither the link is "for profit" or not. The main issue is if the link has any relevance to the material that it is related to. A Wikipedia article (or even a how-to repair manual on Wikibooks) about the Ford Motor Company certainly can have a link to the official corporate website... and be considered a high quality link.
In the case of the UN APDIP books, a link to the original PDF files is certainly relevant as it is the original source material, and you are maintaining academic integrety by noting the original source of the text. In a similar vein, if this offer to have the book on Wikibooks were to happen or even if it were just used as source material to create a very different book, its use as a bilbliographic reference certainly is very justified.
The problem with link spamming is that 300 links for Viagra on a page about Quantum Mechanics is completely irrelvant to the topic under discussion. A sexual health Wikibook, on the other hand.....
I've said for a long time that many links are necessary as bibliographic, reference, and further reading information in a textbook. There is a camp of people who believe that there should be no external links in books whatsoever, as books are supposed to be "self-contained" resources themselves. I think that the "self-contained" metric is simply an alternate way of stating that a textbook is not supposed to link frequently to wikipedia, but instead should maintain it's own narrative. This is, however, neither here nor there.
Because external links are often required in textbooks (or, barring requirement, that they are helpful), it has traditionally be left up to the judgement of the page patroller as to whether or not a link is considered "bibliographical/legitimate reference" or "spam". A potential metric to be used in making such a determination is to ask whether the link in question represents advocacy (and therefore violates NPOV), and also whether or not the link provides financial gain to the target at the expense of Wikibooks. My point above is that proper bibliographic and academic links rarely fail these tests, and won't be deleted as spam.
--Andrew Whitworth
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