Is an answer that says basically "see what book x says about this" considered a link only answer?
Not that i've been seeing lots of these, i was just curious.
Yes, they're effectively the same.
As I understand it, from experience and from reading some of the canonical Meta.SE posts, problems with link-only answers include:
Instead of providing an answer, a link provides directions to a resource that could provide an answer, which is qualitatively less helpful to a person seeking an answer.
No context is provided that indicates to readers whether it is worth their while to follow the link and delve into the resource on the other side of it.
If the link breaks (i.e. the resource is taken down or moved to another URL), the answer becomes completely useless.
The first two issues apply at least equally to online and off-line resources. As TRiG pointed out in a comment, the cost in time and money of obtaining a physical resource is generally greater than that of clicking through to a link, so information that helps the reader decide whether to bear that cost is even more helpful.
The third, link-breakage, doesn't work exactly the same way for print resources, but they could be subject to an analogous issue of books going out of print and becoming harder to access. In addition, the link could be effectively broken immediately for some readers if the printed resource referenced is not available to them or in a language they don't read.