Self comes with a range of documentation:
A good starting point is the tutorial Prototype-Based Application Construction Using Self 4.0, which is a fairly comprehensive introductory tutorial.
After the tutorial, and for more indepth information on Self, there is the Self Handbook, which can be browsed online or downloaded as PDF.
There are a large number of published papers on Self. Note that these cover previous versions of Self and the information in them may no longer be accurate for modern Self and later.