Siemens have a good book for STL, it's called Automating with Step7 in STL, however you being a student it may be a bit costly. the ISBN No. is 3-89578-093-6
To be honest, STL is simple enough but some PLC manufacturers have different ways of doing branches in ladder
perhaps some of the other plc guru's here can point you in the right direction there must be some tutorials on the web, try using Google to search for STL programming.