Here's some books I've found very useful:
Drawing from Line to Life, Mike Sibley, 978-0-9551-5780-6
Your Artist's Brain, Carl Purcell, 978-1-4403-0844-4
The Big Book of Drawing, Andras Szunyoghy, 978-3-8331-5655-7
Keys to Drawing, Bert Dodson, 978-0-89134-337-0
Drawing From Observation, Brian Curtis, 978-0-07-735627-9
I also followed the drawing schedule described in the first 16 chapters of The Natural Way to Draw, Kimon Nicolaides, but I have a very old book that doesn't have an ISBN number. I know it's still offered on Amazon.
Thank you to everyone who took the time to share their favorites