I'm sorry I'm tired of this money is fungible BS. I pay taxes, I do NOT directly fund the library. When I give my money to the government as taxes, I trust that they as an entity will use the money for the benefit of the COMMUNITY not me. So I don't get to say where the money is or is not spent other than to elect my officials.
This is the exact same thing as the bank not being able to tell you that you can't go out and spend all your money on drugs because you still owe them $3000 on your loan. Once the money is no longer in your hands, it is no longer your choice where it gets spent.
The library's mandate is, more than likely, to provide a free, open, public source of knowledge and reading. The source of those materials is up to the library to decide to provide. That may be buying from a local bookseller, it could be from buying from Amazon, or it could be a lending system that is partnered with Amazon and therefore is subsidized by that advertising. The Library has NO, ZERO, ZILCH requirement to answer to every tax paying citizen as to its actions. This may be a democracy or whatever you technically want to call it, but one thing it does not have is a mandate for every government entity to answer to every tax paying citizen. If you have a problem with how tax money is being spent take it up with your representation given to you by the constitution, your elected officials.