Nice article (the Stanford Encyclopedia of Philosophy has many more, btw.), but I like the "Introduction to Lambda Calculus" by Henk Barendregt (one of the people working in Lambda calculus, see for instance the citation to his book "The Lambda Calculus: Its syntax and semantics." in the references), which has the added benefit of being free:

http://www.cse.chalmers.se/research/group/logic/TypesSS05/Ex... (also covers typed lambda calculus AFAIR)