Please make sure to mention both the title and author, and how the book relates to simple living for you!
As inasaba has said, Stoic philosophy is very helpful for simple living, but a book from a different school of thought that I really found quite a bit of value in is How to Do Nothing by Jenny Odell. It discusses a variety of ways to resist the attention economy of social media while also not becoming a hermit, by building small, local, and intentional communities, and setting aside part of your day to just mindfully doing something that brings you joy (The titular โdoing nothingโ) Plus, itโs popular enough that there are usually a few copies at your local library. If thatโs not simple living, I donโt know what is!