When I was a kid I got books from the library too. Then, like most people who enjoy reading, I began buying books of my own. Libraries are fine when you just want to find some books you haven't read before but owning your own books has significant advantages, which I hope need not be restated here.
How this goes for authors depends on how Amazon are purchasing the rights to the ebooks. Having seen what streaming has done to music revenue I can't say I'm too optimistic about this. Some authors will do fantastically well, of course, but the much larger number of people who make a modest income from writing may well see it shrink drastically. The problem is that this doesn't necessarily improve things for consumers. Super-successful authors are often producers of lowest-common-denominator material. If high quality writers who appeal to a much smaller audience are no longer able to support themselves by writing, then they either move downmarket or take up another line of work, which results is a loss of quality for consumers.
The problem with streaming is that proponents treat creative output like a commodity and then point to the laws of supply and demand to justify economic upheaval. But pure supply and demand only applies in cases of perfect competition, where commodity goods are actually perfect substitutes; I have no particular reason to buy oil/ gravel/ corn from supplier X if supplier Y can deliver identical goods at a lower price. This is not the case for creative works.
PS I don't mean this as a dig against Amazon in particular, but there is a potential monopsony problem. I worry about this a bit less with Netflix because the economics of film production and distribution are enormously different than for other media, and Netflix is more like a peer among distributors.