Henk Barendregt, the author of this paper, has an excellent reference book on the untyped lambda calculus called "The Lambda Calculus: Its Syntax and Semantics"

Henk Barendregt, the author of this paper, has an excellent reference book on the untyped lambda calculus called "The Lambda Calculus: Its Syntax and Semantics". I believe he also wrote a book on the typed lambda calculus but I have never read it. This is a good reference text, but in my opinion is not a good introductory text. For a good introduction, I would recommend J. Roger Hindley's introduction to the lambda calculus and combinatory logic entitled "Lambda-Calculus and Combinators: An Introduction". It includes some interesting info on SKI combinator calculus in addition to the lambda calculus.