A songbook of filk songs. Some books include lyrics only; others include sheet music for some or all of the songs. The term hymnal has been used for a number of filk books, particularly those intended for group singing (like church hymnals).
The earliest filk books were produced on duplicating machines and haven't always lasted well. Photocopying is most common today. The books often use comb binding so they can lie flat when opened.
Early filk books paid little attention to copyright. Since the 1980s, publishers have been more careful about getting permission to include songs.
The following is a very incomplete list of filk books.