This is both a book and a journal at the same time. Although it is primarily written by you, every once in a while it provides interesting insights from various studies on how expressing gratitude can improve your well-being.
It's also an excellent tool for developing a consistent gratitude habit. It took me more than a year to complete; I had some pauses, but I kept coming back to it, and in the process, I built another good habit.
If you're a Kurzgesagt fan and are interested in this kind of topic, it's definitely for you. I was just looking through some pages I filled out months ago, and it felt so heartwarming—that's another great benefit to consider.