Several collections I have are highly recommended for reference:
1905: Seventy Scottish Songs Selected and arranged by Helen Hopekirk; ISBN 0-486-27029-7
1916: One Hundred English Folksongs Edited by Cecil J. Sharp; ISBN 0-486-23192-5
1927: The American Songbag by Carl Sandburg; ISBN 0-15-605650-X
1934: American Ballads and Folk Songs by John A. Lomax and Alan Lomax; ISBN 0-486-28276-7
1939: Folk Songs of Old New England Collected and Editied by Eloise Hubbard Linscott; ISBN 0-486-27827-1 [reprint of 1962, Second Editon, made in 1993]
1960: Songs of Work and Protest by Edith Fowke and Joe Glazer; ISBN 0-486-22899-1 [1973 corrected version]
1960: The Folk Songs of North American in the English Language by Alan Lomax; ISBN 0-385-03772-4
1962: A Russian Song Book Edited by Rose N. Rubin and Michael Stillman; ISBN 0-486-26118-2
1962: Songs for Pickin' and Singin' Selected and arranged by James F. Leisy; ISBN 0-449-02457-095
1965: Folk Songs of the Southern Applacians (Second Edition) as Sung by Jean Ritchie; ISBN 0-8131-0927-2
1966: Songs and Ballads from Nova Scotia Collected by Helen Creighton; ISBN 0-486-21703-5
1975: The Best Loved Songs of the American People by Denes Agay; ISBN 0-385-14006-1
1991: Folksongs from Africa selected and arranged by Malcolm Floyd: ISBN 0-571-51230-5
1991: Folksongs from the Far East selected and arranged for voices and keyboard by Peter Gritton; ISBN 0-571-51203-8
1992: Folksongs from Eastern Europe selected and arranged for voices and keyboard by Ken and Jean Bolam; ISBN 0-571-51308-5
1992: Popular Irish Songs [1808-1914] Edited by Florence Leniston; ISBN 0-486-26755-5
1993: Folksongs from India selected and arranged by Peter Gritton; ISBN 0-571-51372-7