''Syntax and Semantics of Programming Languages'', by Ken Slonneger and Barry Kurtz.  Another OnlineBook on programming language design; this one has lots of material on semantics, moreso than many other programming language texts.  Many semantic models are covered, including AxiomaticSemantics, DenotationalSemantics, OperationalSemantics, ActionSemantics, etc.

Available online (in PDF form) at http://www.cs.uiowa.edu/~slonnegr/plf/Book/

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CategoryOnlineBook