Toki Pona: The Language of Good
Toki Pona: The Language of Good is the foundational resource of modern toki pona. It was written by jan Sonja to describe her way of speaking with toki pona, and published in 2014. Since then, it's been translated to French, German, and Esperanto.
It's also known as "lipu pu", "lipu Pu", or just "pu".
It's available for purchase at https://tokipona.org/.