Setting a cover in one typeface
Typography
Using a single family on a book cover is a constraint that makes every other decision clearer.
For a recent publication I set the whole cover in one typeface. The title, the author's name, the series line and the small print all share a family. It sounds restrictive, and that is the point.
Let size and weight do the work
With one family, hierarchy has to come from scale, weight and spacing. I try the title at three sizes before I commit and view each at thumbnail size, because many people will first meet the cover as a small image online. If the title still reads at the size of a postage stamp, the structure is sound.
Weight does the rest. A heavy title with a light author line feels confident; reverse them and the cover becomes quiet. Neither is wrong, but the choice should match the book. I keep the secondary text to two sizes at most, so the page does not turn into a list of competing voices.
Spend the saved decisions on spacing
Without a second typeface to select and pair, I have more time for the details that matter: the gap between lines, the margin from the trim edge and where the eye lands first. Printed samples help, since a margin that looks generous on screen can feel tight on paper.
The result is rarely dramatic. It tends to look settled, which is what I want from something a reader will hold for hours.