Man, I went down such a rabbit hole trying to find 'The Lottery' by Shirley Jackson last semester for a class. It's a short story, not a full book, which is the first thing to know. Public domain stuff can be tricky, but a lot of libraries have it in their digital collections through apps like Libby or Hoopla. That's how I got it free and legal.
Honestly, just searching the title on its own pulls up a ton of sketchy PDF sites. I'd avoid those. The New Yorker actually published it first in 1948, and their website sometimes has older pieces accessible. Your absolute safest bet is to check if your local library partners with a digital service. Took me five minutes to sign up with my card and borrow it. The whole process was easier than dealing with pop-up ads on those free sites, and you're supporting libraries, which is a nice bonus.