EUCAST is increasingly becoming the dominant international standard for various reasons and its procedures and documentation, including breakpoint tables are available online for free.  For a recent overview of EUCAST, see this Journal of Antimicrobial Chemotherapy 2014 lecture (free access).

Turnidge and Paterson, Setting and revising breakpoints, CMR 2007 is an excellent reference covering some key concepts (free access).

Basic concepts- Kahlmeter lecture 2014: 

• Wild type distributions
• ECOFFs (Epidemiological cutoffs)
• Phenotypic vs genotypic antibiotic resistance

AST summary – Kahlmeter lecture 2014:

• How is the minimal inhibitory concentration (MIC) determined?
• What are proxy methods  for MIC determination?
• Clinical breakpoint determination vs ECOFF

Other references (see also the EUCAST website )

• EUCAST-Expert-rules-version 2- essential reference for intrinsic resistances in Gram positive and negative bacteria. Where a species is intrinsically resistant to an antibiotic, there is no rationale for testing susceptibility
• CLSI vs EUCAST Turnidge– Evaluation of the key differences between the two ISO-standardised methods for AST –