Shneur Zalman of Liadi published the Tanya in 1796, and it is still the book the movement teaches from. Its formal title, Likkutei Amarim, means collected sayings, and it is organised as a manual for a particular kind of person rather than as a systematic theology. That person is the beinoni, the intermediate one, who is neither a saint nor wicked. The appetites are all still present and are never expected to go away, but the beinoni does not act on them, does not speak from them, and does not let them occupy the mind. The emotional centre of the book is the claim that this condition is available to anyone who wants it, whereas sainthood is not.
The method is where Chabad separated from its neighbours. Much of early Hasidism worked on fervour directly, through song, movement and attachment to a rebbe whose own state carried the community with it. Chabad routed the same fervour through the intellect. Chochmah is the flash of an insight, binah is the working out of it, and daat is the settled knowledge a person actually lives from, and the initials of the three give the movement its name. Long contemplative dwelling on a divine idea is expected to produce feeling, and feeling arrived at that way was held to be more durable than feeling raised directly.
The book also carries the school cosmology, in which created things have no independent existence at all and their apparent solidity is a concealment of the divine reality that is all there is.