Looking at the early returns, they're kinda missing the point about empowerment.
I want to draw people in, have the imagery show ACTION (see the three covers I selected in the "Design Inspiration" section.) Like a sculptor making a 3d printer out of a block of granite. Or a wizard waving his wand to make one appear. Or Mister Science with an illuminated light bulb on his hat with tools in his hand busy putting one together. Not necessarily one of those, but I hope you get the idea -- ACTION, CREATION, INNOVATION. Also, since you should include at least some portion of a printer on the cover, it should really look like a 3D printer, not some nondescript or weird "what is that?" machine. There are any number of examples out there to use as inspiration. If you're not sure, search on "Ender 3" or "Prusa MK3" or "CoreXY". The last one is probably your best bet graphically. I'm adding some examples in the Files area. Illustrations please, no photos.
This is intended for an 8-1/2 x 11 inch format, full color with bleed. This book is all about empowerment, being able to tackle a challenging project with confidence. Also, it's important that the cover convey at a glance what the book is about -- Designing and building 3D printers (It's NOT about USING them, there are already lots of books like that!). It's also key that it looks like something that anyone can understand, not elitist or egghead.