Fake Book Cover

This book does not really exist, and is unlikely to do so, so let's get that out of the way.
Having realized that I will probably be randomly self-publishing things for many years to come, I finally decided that I need to address one of the glaring holes in my artistic skillset--graphic design.
I'm not great at it. But I'm gonna be doing my own book covers as T. Kingfisher for quite awhile. So I spent awhile on Pinterest pinning covers I like or found interesting or eye-catching, and then it occurred to me that there is pretty much no artistic skill you learn simply by looking at examples of it, so I started fooling with fake book mock-ups. My buddy Daniel Solis, whenever anybody drops a particular complex or weird line into the conversation, will simply say "...a memoir," and so that turned into fake memoir covers and so forth.
I dunno, it sort of works large but the contrast between the green and the brown isn't strong enough at the small size, so if it was kindle-thumbnail-size, it'd just look too dark.
If anyone has any suggestions for really good entry-level design books, I'd love to hear 'em.