I'll start!

I really like The Enchiridion by Epictetus, which is a very short introduction to Stoic philosophy. Stoic philosophy ties in closely to simple living for me because it allow me to focus on the things that I can control and that matter to me, letting go of those things that I cannot control. Meditations by Marcus Aurelius is another Stoic text, longer than the Enchiridion, that gives insight into the life of someone living their Stoic values. It's a series of journal entries with much insight into daily challenges, and I have taken away a lot from it. Here's one of my favourite quotes:

If you seek tranquility, do less. Or, more accurately, do what's essential – what the logos of a social being requires, and in the requisite way. Which brings a double satisfaction: to do less, better. Because most of what we say and do is not essential. If you can eliminate it, you'll have more time, and more tranquility. Ask yourself at every moment, "is this necessary?"

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 'doing nothing') Plus, it's popular enough that there are usually a few copies at your local library. If that's not simple living, I don't know what is!