A blog entry discussing residual finiteness and Hopfian groups.
A blog entry discussing Marshall Hall's theorem for free groups.
A different blog entry discussing residual finiteness.
A blog entry discussing subgroups separability.
Specifically sections 2 and 3 of Henry Wilton's Notes
The comprehensive text that gives full and extensive detail. Available in all good libraries. Don't buy it with your own money.
Wise and Hruska's general account of the dual cube complex machinery.
Michah Sageev's PCMI notes. A good exercise is to realise that the language of pocsets gives the same outcome as the wallspace terminology.