I prefer the first cover. In both cases, however, I think the biggest detractor is the typography. I don’t think yellow is such a bad choice for the title in general, but it definitely works less in the second cover than in the first. It does evoke caution or danger and, against black, yellow is attention grabbing and legible. It’s also worth considering that primary yellow has a tendency to read as cheap/low-budget to consumers. You could also switch out the yellow with white.
Is the title typeface squashed or stretched in any way? It seems to be at first glance, but that could just be because the title typeface proportions are so much different than those of your serif body text. Speaking of the body text, the column could use a bit of finessing. You might want to try justifying the text, making it the same width as your title, or turning off hyphenation. On the second cover, your title shouldn’t be broken into two lines. I’d lower the type size and make sure the break between your title and the first paragraph is the same height as the break between the first and second paragraphs.
Because you have to go with a smaller type size on the spine, for legibility reasons I recommend using all caps and increasing the letter spacing a hair.
This is just a style manual note, but the title of your book should be italicized when referred to in your body text (i.e. “Brutal Adaptation examines the implications of letting fear…”).
Last but not least, congratulations on your book.