Some useful documents that give an introduction to structural (small-step) operational semantics:

• A Structural Approach to Operational Semantics – Gordon D. Plotkin. Lecture notes for a course on structural operational semantics.
• A Syntatic Approach to Type Soundness – Andrew K. Wright and Matthias Felleisen. Description of evaluation contexts.
• The Origins of Structural Operational Semantics – Gordon D. Plotkin. Disscussion of the history of structural operational semantics.
• Steffen van Bakel’s old Applied Operational Semantics course notes
• Andrew Myers lecture notes