A brief motivation is in the first few sections of the readme.
You need to follow the install guide.
The above will also point you to the configuration guide.
The specifics of the main design are in this document.
The Bitcoin scripts and transaction design are illustrated here.
Further modifications to the design for fees are explained here.
Some lower level design documentation:
The JSONRPC specification details the client-server communication.
The logic of the state machine used to ensure correct program logic is here.