Lambda calculus lies at the very foundation of computer science. Besides its historical role in computability theory, it has had significant influence on programming language design and implementation, denotational semantics and domain theory. This book is written from a systems perspective, emphasizing the practical relevance of many of the key theoretical ideas.
This is a nice introduction to lambda calculus, combinatory logic and the like topics. It is based on Hankin's undergraduate course at Imperial College London. It is not self-contained; there are many references to the more comprehensive (and much more expensive) textbooks by Barendregt and by Hindley and Seldin. I wish I had read it in college.