The Doctrine of the Trinity: No Christianity Without It

A good overview: The Doctrine of the Trinity: No Christianity Without It – Kevin DeYoung:

The two key words here are essence and persons. When you read “essence”, think “Godness.” All three Persons of the Trinity share the same “Godness.” One is not more God than another. None is more essentially divine than the rest. When you read “persons”, think “a particular individual distinct from the others.” Theologians use these terms because they are trying to find a way to express the relationship of three beings that are equally and uniquely God, but not three Gods. That’s why we get the tricky (but learnable) language of essence and persons.