There are a series of tutorials developed and maintained by MATLAB developers that you can use for every aspect of the Simulink.
for example: "SimPowerSystems User's Guide.pdf" and the latest version I could find had "Revised for Version 5.8 Release 2013a" and are almost online versions.
For MATLAB version R2016(I think) and newer the name of the "SimPowerSytems" has changed to "Simscape" and so is the name of the tutorials. Now it is something like:
The web page for the PDF format documentation is: