It's nothing in isolation of your single act. Maybe. In aggregate it of course deprecates the library(they have e-checkouts already but you're talking untracked piracy). I like libraries; I think they're mostly performative in small towns and providing social services in cities rather than functioning as actual libraries. Kinda sad.

I pirate all my books btw, just saying if everyone did it ofc there'd be(already is) impact. Easily smartphones+kindle is worse though.