In terms of the 4-document model, I find a lot of software systems (libraries and frameworks), while they are good at providing tutorials and an API reference,

In terms of the 4-document model, I find a lot of software systems (libraries and frameworks), while they are good at providing tutorials and an API reference, they are a lot poorer at providing an Explanation. What I want to know when learning a new software system are what are the main classes/concepts in it and how they relate to each other.