Documentation

Clean Language Report

There are several books about Clean.

Functional Programming in Clean is a practical book in progress intended as an introduction to functional programming in Clean for undergraduate students. It is available online online.
Functional Programming and Parallel Graph Rewriting is a book that describes the theoretical foundation and implementation of Clean. It has been published by Addison Wesley in 1993.
Programming in Clean is a nice Clean book for beginners. It is available online.