The user had no copyright claim on that work, but he/she did have a reasonable expectation that a "purchased" good was hers to dispose of as she saw fit after completion of the sale, within the confines of the law. What people are objecting to is Amazon's redefinition of the rights of the seller at the expense of the rights of the buyer.
It's also interesting how similar this is to the previous round, a century ago: the first-sale doctrine arose precisely to put an end to shenanigans publishers were trying to pull where they claimed they were only "licensing" a copy of the text to the person who purchased the dead tree. It reasserted the property rights of the purchaser, which companies now are trying to erode with the same "just licensing" shenanigans.
On which subject in particular do you want pointers to additional reading?
(I'm going to assume that for basic background information, such as the history of the "first-sale doctrine", you're as capable of using Google and Wikipedia as anyone else.)
