Dov Ber wrote nothing for publication. What survives is a set of teachings taken down by disciples and issued after his death, the principal collection appearing in 1781, and even there the material is fragmentary: short, dense, often built around a parable and broken off before it is fully worked out. Anyone reading him is reading notes of what people remembered him saying.
The recurring idea is bittul, the nullification of self-existence. Creation, on his account, does not have independent being at all; what looks like a world is the divine speech continuously sustaining it, and a person who grasps this stops experiencing himself as a separate thing standing over against God. The practical upshot is a doctrine of prayer in which the aim is to get the praying self out of the way, and a doctrine of the tzaddik as a person through whom divine influence passes because there is nothing in him obstructing it.
His parables are the part that travelled. The father who steps back so that the child will walk toward him, and who is not withdrawing but drawing; the seal that leaves an impression which is the reverse of itself, and the world as such an impression. These are used across later Hasidic literature by writers who take them as common property. The teaching that reshaped a movement survives entirely at second hand, which is a fact about him worth stating alongside the doctrine.