Tanya תניא
The foundational book of Chabad: the anatomy of the soul and a practical path for the «intermediate» person.
Rabbi Shneur Zalman of Liadi wrote the Tanya over twenty years, in answer to questions his hasidim brought him, and published it in 5557. Its second title, «the book of the intermediates», is the key: it is written neither for the righteous nor for the wicked but for a person who struggles and never quite wins.
Its central idea is that every Jew has two souls, divine and animal, and they do not merge. The beinoni never sins in deed, speech or thought, yet feels the pull toward it all his life. That, says the Tanya, is not failure but the norm and the goal: holiness is measured by conduct, not by feeling.
The second part sets out the teaching that the world is continuously created by speech and that «there is nothing besides Him». The Tanya gave hasidism a systematic language and was the first hasidic work built as a treatise.