In a feeble attempt to return the discussion to its original focus, the primer I used was Theodore Walker’s Planting Design (1991). It was geared toward landscape architects, but would be very straightforward for an instructor teaching motivated non-designers. The only real drawback a couple of decades later is that the illustrations and drawings looked dated even when I was in school.
Since your course will be directed to an audience that should be familiar with the botanic characteristics of the plants they will be likely to use, you should have a lot of fun focusing more on the artsy side of planting design than you’d otherwise be able to with a bunch of first-year college students. Your biggest obstacle (I think) will be getting your students to communicate graphically in a short course without wasting too much teaching time on the subject.