Two suggestions to add to the many excellent ones already here:
Arthur Wesley Dow’s Composition: A Series of Exercises Selected From a New System of Art Education [0] is a wonderful, wonderful book in the public domain – recently submitted here but didn't get much attention. It is well worth spending a lot of time studying its exercises and examples.
[0] https://publicdomainreview.org/collection/dow-composition/
My second suggestion is more general but it worked wonders for me. I had a teacher years ago whose take-home assignments consisted of extremely rudimentary drawing exercises, starting with drawing a series of straight lines in parallel across a page, then moving on to squares, circles etc but always basic geometric shapes. His point was that drawing is, at its most basic level, a physical skill like many others which require physical and neurological adaptations that can only be improved with practice. In addition to the creating these lines and shapes, one was obliged to pay strict attention to their relation to other marks on the page – your circles could not overlap, for example, and your straight lines must run parallel and start and end at the same page border. The lines had to start and end very definitely at the correct point. He was also keen to examine and correct each student's physical stance in relation to the easel, the light, and the subject, and insisted we draw from the shoulder, not the elbow or wrist, as much as possible. The goal was to train one's whole body to gain full control over where the instrument marks the page.
For some students this practically remedial approach seemed almost insulting and a few would balk at it, but it was just incredible how much even very mediocre drafting skills improved when these exercises were applied diligently, including my own. I'm sorry that I don't know a name for this pedagogical system or a book that explains it.