Re: So...
The local library in my county lists 165,128 volumes in its collections. While 600,000 may sound like a considerably greater amount, it will probably make little practical difference. While Amazon has greater span compared to my local library system, it also has a much larger customer base. Even assuming only a fraction of their current customers sign up, there will still be a far greater number than live in my area and make use of the public library. With them will come far greater variations in reading taste and selection.
Also, Amazon currently has a big incentive to increase returns on this new effort. They will almost certainly be pushing people to buy books from them based on the data gathered from the service. While this is a pretty good business plan, I would not have to deal with this from a public library. They are apt to push other books on me, it still comes to the low price of my tax contribution.
On the other hand, public libraries offer services such as inter-library loans, public meeting rooms, and free events for kids. None of these are likely to be offered by Amazon with this new paid-for service. I am not sure how much of that applies to you as you are almost certainly in a different locale, but I would guess that at least some of it holds true.