The Mind Illuminated - Comprehensive meditation guide that comes at it for an non-theistic/religious/spiritual perspective. Everything you ever wanted to know about meditation without having to go through books about Buddhism or meditation which are generally very spiritual and can be difficult to read.

The Attention Revolution - In-depth guide to how to meditate without too much fluff. It basically directly goes into all the details of meditation practice as a thing you do and how to do it. Also very comprehensive and jumps straight into how to meditate.

The Practicing Stoic - Great overview of Stoicism with a lot of quotes from all the well-known Stoics compiled into one Book. I think it's a great introduction for anybody who isn't quite sure about Stoicism or what it's about, but it's also a great book to go back to as somebody more well-versed in it to refresh the concepts.