Charles Petzold (who wrote "Code") has written a very good book which goes through Turing's paper in small sections: The Annotated Turing: A Guided Tour Throug

Charles Petzold (who wrote "Code") has written a very good book which goes through Turing's paper in small sections: The Annotated Turing: A Guided Tour Through Alan Turing's Historic Paper on Computability and the Turing Machine