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"
...