If you don't mind reading ebooks as opposed to physical books, I'll start posting when Amazon lets me give my books out for free. Every few months, it allows me to give out the ebook version for free for five days. The majority of my books are psychology self-help books. One of them is published traditionally and available more widely (the one below), and the rest are self-published through Amazon. Here's the one that's free right now:
The Serotonin Book: How to Maximize Serotonin Levels Naturally
https://www.amazon.com/dp/B0BYNNH484
The Serotonin Book: How to Maximize Serotonin Levels Naturally
https://www.amazon.com/dp/B0BYNNH484