Direct citations shall be provided as required throughout this webpage however there are certain works that will invariably be checked against for anything written and serve as the basis for the author’s knowledge of lichens. Here they are presented in order of the frequency I consult them.
The Consortium of Lichen Herbaria (CNALH) website – Indispensable for it’s descriptions of species up to date taxonomy and repository distribution data from herbariums around the world.
https://lichenportal.org/portal/
Lichens of North America by Irwin M. Brodo, Stephen Sharnoff, and Sylvia Duran Sharnoff – The single best reference for Lichens covering the Americas.
Keys to Lichens of North America: Revised and Expanded by Irwin M. Brodo – Takes the already excellent keys in the original volume and adds addtional content as well as covering additional species.
Lichen Flora of the Greater Sonoran Desert Region Volumes I, II and III by Thomas Nash et al. – Oh if only Colorado snuck into the region. An authoritative treatment of lichens in the area, luckily overlapping greatly with Colorado. Indispensable for the most technical of queries. Some information in the book is available on CNALH.
Steven Sharnhoff’s Website – Incredible photos identified by an expert in lichens.
https://www.sharnoffphotos.com/lichens/lichens_home_index.html
Ways of Enlightenment Website – Incredible photos identified by an expert in lichens.
https://www.waysofenlichenment.net/

Additional References:
Clements, Frederic E. and Cornelius L. Shear. 1931. The genera of Fungi. Illustrated by Edith S. Clements. New York, The H.W. Wilson Company. http://www.biodiversitylibrary.org/item/26605. MBLWHOI Library, Woods Hole. (DOI: http://dx.doi.org/10.5962/bhl.title.5704)
Nash, Thomas H. III, Ed. 1996. Lichen Biology. Cambridge University Press. 303 p.
A guide to the study of lichens, by Albert Schneider.
By: Schneider, Albert,
Edition: Second edition.
Publication info: Boston,Knight and Millet,1904.
Contributed by: NCSU Libraries (archive.org) http://www.biodiversitylibrary.org/item/63169
The lichen flora of the United States, by Bruce Fink … completed for publication by Joyce Hedrick.
By: Fink, Bruce, – Hedrick, Joyce.
Publication info: Ann Arbor,University of Michigan Press1935.
BHL Collections: MBLWHOI Library, Woods Hole
http://www.biodiversitylibrary.org/item/26569