The first few pages of the book.

A good brief intro How to do plain programming in λ calculus

An excellent review by Orrin Frink that would serve well as an introduction.

An Introduction to Lambda Calculus and Scheme by Jim Larson (From Church’s perspective to programming language)

The impact of the lambda calculus by Henk Barendregt (tells the origin of “λ”!)

A Tutorial Introduction to the Lambda Calculus by Raúl Rojas

The Lambda Pattern by Dorin Sandu, Dwight Deugo

Church’s type theory and logic

A short biography of Alonzo Church with some technical information.