These readings are cultural, to help you develop some perspective on the role of functional languages and functional programming in real-world software development. Many of the articles refer to Lisp, a close cousin of Scheme.
Paul Graham's site has number of other articles discussing advantages of using Lisp in programming projects.
Successful Scheme, an article from Unix Insider about Scheme and some of its real-world uses.
The Functional Programming in the Real World website, with pointers to a slew of real-world projects that use functional languages.
Schemers.org, the online home for all things Scheme.
Material related to the optional lecture on lambda-calculus (programming just with lambdas):