You can view the the documentation on separate  SharpSetup documentation page and in the documentation section for additional information (eg. SharpSetup tutorial, .NET prerequisite modules, etc.).

Apart from the above mentioned documentation from time to time I publish information in form of short articles in SharpSetup blog that describe SharpSetup concepts, structure and features. Here is a list of those articles:

Overview

What is SharpSetup

First SharpSetup screenshots

Structure

SharpSetup installer structure

Installer GUI design

The Package project (WiX based)

SharpSetup bootstrappers

Multilanguage installers

Assembler project

GUI specific articles

Using installer dialogs

Step lifecycle

Controls overview

FeatureTree control