If a PPL holder took a few snaps while flying, and now these pictures are used in a book, with the author, the publisher and maybe even the photographer making money, wouldn't that be "aerial work", in retrospect? And thus, retroactively, make the flight illegal?
When I'm flying and my passengers are taking pictures, I always make sure to tell them their pictures cannot be used commercially.
(In the Netherlands, a secondary issue is that formally you need permission from the MIVD - the military intelligence unit - before even taking a camera into the air. This is because of some cold-war statue that was never revoked. Nobody really cares, but publishing aerial pictures for which you did not get permission could technically cause a lot of trouble.)