Reference sources for lichen identification, descriptions, habitat details and synonyms include:
The Lichens of Great Britain and Ireland - edited by Smith et.al, 2nd edition 2009
The Lichen Flora of Great Britain and Ireland - edited by O. W. Purvis, 1992
Lichens An Illustrated Guide to the British and Irish Species - Frank Dobson
Lichens of North America - Brodo, Sharnoff & Sharnoff, 2001
The Macrolichens of New England - Hinds & Hinds, 2007
Websites and resources:
The British Lichen Society: Publications
LichenIreland: References
Summerfield Books
NHBS Bookstore
BLS species maps
BLS - chemical tests
BLS Lichen Taxon Dictionary
BLS Lichenicolous Fungi
Revisions of British and Irish Lichens (LGBI 3)