Page "Lions' Commentary on UNIX 6th Edition, with Source Code" Paragraph 1

For many years, the Lions Book was the only Unix kernel documentation available outside Bell Labs.

Although the license of 6th Edition allowed classroom use of the source code, the license of 7th Edition specifically excluded such use, so the book spread through illegal copy machine reproductions ( a kind of samizdat ).

It was commonly held to be the most copied book in computer science.