Colin Tudge's amazing book, The Variety of Life is not a field guide, but it is a survey and celebration of all the things that have ever lived. I cannot recommend this book enough.
Newcomb's Wildflower Guide truly is the most effective key that I have ever used. Positively genius.
A Field Guide to Trees and Shrubs by George Petrides is the best guide to woody plants in my region.
Jean-Baptiste Lamarck's first widely distributed field guide was Flore Francaise.
Lawrence Griffings paper about Richard Waller's rediscovered key can be found in its entirety here.
The National Library of Medicine has this amazing series of blog posts on the history of herbals and floras, books that were the precursors to field guides in medieval Europe.
Cornell Ornithology Lab's Merlin app really is amazing and is available to download for free.
