It was a few pages back, but the pertinent link is
here. The principal author Ronald Drew was at the very least indicted for sex crimes against children, although I can't find anything that conclusively says whether he was acquitted or convicted. However the books' editor Martin Swithinbank very definitely served several years in prison for sexually assaulting young boys at his house and was deported to the UK after serving his sentence.
This article which focuses specifically on Swithinbank's long history of involvement with pedophilic organizations and publications (read the whole thing if you want, I guess; it's boring and awful), has this to say about Book Explorers, Inc - the company which published the two "art books" in question:
So the books were authored by at the very least an accused pedophile, edited by a convicted child rapist, and even published by a company specifically aimed at a pedophile audience.
Are they books "child porn"? Not legally. But that's the entire point: the books were made and sold with the intent of providing pedophiles with photos of naked children that they could use
for pornography but which they could legally own because they photographs aren't explicitly sexual.
And if you think that's a stretch, this isn't even an outlier case. Only a few years ago
a company in Canada was busted up which sold "naturist lifestyle" videos (exclusively containing young boys), and hundreds of people from its mailing list worldwide were arrested, some of them having been found with much much worse material besides.
This argument is identical to the one used to defend Jackson's owning these books. But the context of the material having been made by pedophiles primarily for pedophiles, changes things up a bit; because if that's the case, then these
aren't some general-popular-interest random art-books that anyone looking for photography of children might easily have in their art-book-library, as has been argued heretofore. They're the kinds of things that were consciously made to appeal to a specific taste and have to be specifically looked for to be found.