The author mentioned that he'd be comfortable with the CC-by licence, but requests explicit autorisation in the case of the CC-by-sa licence. I reckon that this is because it's not clear whether the whole book would be considered to be a derivative work of the image or not (I think that it's obviously not the case, but it's only my opinion and others think otherwise).
For the credit line, it depends on where credits are given. On a "photographic credits" page, it's easy because lots of space is available. Immediately under a photograph, the credit line should be concise (which is why CC-by-sa works much better than the GFDL in this case). Hence my question: the thing should be informative, advertise Commons efficiently, yet be non-intrusive and concise. Something like "(c) Foobar, Cc-by-sa, Wikimedia Commons" "Photograph Foobar (Wikimedia Commons), Cc-by-sa" "Photo Foobar, pub. Wikimedia Commons, Cc-by-sa" ...