The most essential literature to read before starting to code any McStas is
http://www.mcstas.org/documentation/manual/mcstas-manual.pdf McStas manual
1 (General introduction - optional)
3 (Monte Carlo introduction - optional)
4 (On running McStas)
5 (kernel and meta-language)
Appendixes A and B could also be relevant.
http://www.mcstas.org/documentation/manual/mcstas-components.pdf McStas component manual
is mostly good to have as reference information. Chapter 2 is the same as chapter 3 of the ordinary manual.
It may also be helpful to perform a few simulations as a McStas user - for instance via the McStas tutorial, available via the mcgui -> mcdoc menu