I tried to find some books that looked helpful that can be read at no charge. Here are some that are on openlibrary. I haven't read these but I looked at each briefly and these looked like they might be helpful. I could look more thoroughly but I thought I may as well share this list even though it's not as carefully curated as it could be.
If you have other suggestions for books please feel welcome to add them in the comments.
The Book of Kindness: Power of the Gentle Path compiled by Mary Lou Cook, Jan Lurie, Richard Polese. A compilation of quotes about kindness.
Altruism: The Power of Compassion by Matthieu Ricard. Sortof a scientific-ish approach discussing some topics related to altruism. Author seems to have a background in genetics science and also in Buddhist spiritual teachings.
The Power of Kindness: The Unexpected Benefits of Leading a Compassionate Life by Piero Ferrucci. Discusses some different aspects of kindness and related topics.
Capitalizing on Kindness by Kristin Tillquist. Being kind is compatible with and helpful for success in business.
The Fragrance of Kindness: Giving the Gift of Encouragement by Cheri Fuller. Christian. Lots of stories and quotes. Designed to be pretty to look at. Has exercises you can do.
there doesn't seem to be anything here