Peter Smith has a pretty good [handout on Tennenbaum's theorem](http://www.logicmatters.net/resources/pdfs/TennenbaumTheorem.pdf) that I found useful when learning that material. As others have mentioned, Richard Kaye's _Models of Peano Arithmetic_ is the go-to reference work here. Kossak and Schmerl's _The Structure of Models of Peano Arithmetic_ is meant to give the state of the art circa 2006, but you probably won't need this one.