I very much recommend The Essential Hoof Book by Susan Kauffmann and Christina Cline. It includes very useful explanations and photographs:
I very much recommend Care and Rehabilitation of the Equine Foot by Pete Ramey with plenty of articles written by Dr. R. Bowker and other experienced scientists. Includes also interesting information about nutrition as well. Delves deeper into the topic.
General approach to trimming according to Dr. Bowker: https://aaep.org/horsehealth/bare-foot
Lecture by Dr. Bowker – „The hoof walls are mere decoration“: https://www.youtube.com/watch?v=DsnJivQ0OCU
Dissections of horses hooves The Study of the Equine Hoof, Lindsey Field: https://www.patreon.com/HoofStudies/overview
Uniform sole thickness as trimming guide: https://www.naturalhorsetrim.com/Michael_Savoldi_uniform_sole_thickness.pdf
Anatomy of the horses hoof by Paige Poss: http://www.ironfreehoof.com/
Brian Hampson´s study about wild horses in Australia: https://www.agrifutures.com.au/wp-content/uploads/publications/11-140.pdf
Trimming according to Marjorie Smith: https://www.barefoothorse.com/index.html
Chris Pollitt laminitis study: http://horsefx.com.au/wp-content/uploads/2015/07/Chris-Pollitt-Laminitis.pdf
Chris Pollitt´s lecture about feral horses research in Australia Part 1: https://www.youtube.com/watch?v=1reZR1FCn_s
Chris Pollitt´s lecture about feral horses research in Australia Part 2: https://www.youtube.com/watch?v=Uu5Jd_DyAjU
Lecture by Dr. Debry Taylor – Is the horse hoof a smart structure?: https://thehorse.com/155327/is-the-hoof-smart-adaptability-of-the-equine-foot/