I'm new to programming languages theory and I'm seeking for a good resource on a resource for formal semantics  of programming languages. Specifically looking  for structural operational semantics. I got some book recommendations. But I'm looking for a resource in a  more introductory level. Especially tutorials, web-site and free book recommendations are  welcomed.
Formal Semantics of Programming Languages
reference request;pl.programming languages;semantics;formal systems
Looking at the operational semantics course at my University gives the following three:Andrew Pitts's lecture notesGordon Plotkin's Aarhus notes on structural operational semanticsMatthew Hennessey's The Semantics of Programming LanguagesOhad.