The way of Zen by Alan Watts.

You can absolutely find people more qualified for book recommendations on the subject than me, but here are a few:

"Confession of a Buddhist Atheist" by Stephon Batchelor. Useful to me for understanding the cultural context of Buddhism.

Insight Meditation by Joseph Goldstein. I just find Goldstein's style the easiest to comprehend in a field that often comes across as frustratingly esoteric

Wherever You Go There You Are by Jon Kabat-Zinn. I thought this book is interesting because it draws parallels to Western authors, particularly Thoreau.

Sam Harris's "Waking Up" app may also be worth a listen. Besides the actual mindfulness practice, there are a lot of discussions on theory and he has conversations with a variety of people that come to the topic from different perspectives.