Penstemon resources in print
Flora of Colorado by Jennifer Ackerfield
Penstemon of Southeastern Colorado by Don and Donnie Barnett
- species covered are relevant to many areas east of continental divide from Colorado-Wyoming border to Southeast New Mexico
- https://www.ethicaldesert.com/product-page/penstemon-of-southeastern-colorado
Flora of the Four Corners Region by Heil, O'Kane Jr, Reeves, and Clifford
Penstemons: The Beautiful Beardtongues of New Mexico by Heflin, et al
Flora Neomexicana by Allred and Jercinovic
The Heart of Penstemon Country: A Natural History of Penstemons in the Utah Region by Stevens, et al
Northwest Penstemons: 80 Species of Penstemon Native to the Pacific Northwest
by Dee Strickler and Anne Morley
[Cost: $18.50 including postage. To get a copy of the book, use this form https://drive.google.com/file/d/1B6LK6PNCC0ov9zfdx35np-_oJJt95l9b/view]
Please add more in the comments!