@Griffin-McPherson going to address one of your questions: Does this work as a book cover?
In its current layout, it's not the best composition for a book cover. Here are a few reasons why:
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Readability. As @marek-halko pointed out, value and contrast have a huge bearing on readability. A book cover should be clear when viewed up close (holding the book) and zoomed out (for example, as a thumbnail image on Amazon or on a book shelf on the other side of the book store). The more readable a cover is, the more likely it is to catch attention, and the more successful it will be.
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It's busy. As a book designer, I would question where is the best place to put the title -- it's going to obscure a good portion of all that city detail. So part of the illustration will be lost. Also, having a busy scene with a lot of contrast behind a title will make it difficult to read; it won't stand out. And where will the author & illustrator names go?
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The characters. Making the characters a different color than the background is great; it helps them stand out. But placing them so far down on the page visually gives them less importance than the city, which dominates the composition. On a book cover, a reader will connect more with a character than an environment, if both are present on that cover. A reader will also want to get a sense of who these characters are and what their story is about. You've done a great job of making these characters look apprehensive and/or overwhelmed by the city, but what are they looking at? What are they scared of? That part of the storytelling isn't clear for me.
If you want to use this illustration as a book cover, this book designer's recommendation is:
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Simplify and reduce the contrast in the background.
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Give the characters greater prominence. Increase their size and move them up a bit from the bottom of the composition.
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Design the cover so that there is space for a title and bylines that won't obscure important parts of the illustration.
Here is a quick example of cover design with a side by side comparison of this illustration as-is, and with some of those suggestions implemented:
(Hope you don't mind that I took a screenshot for this example. This isn't the best cover design, and probably not the best font choice -- just a visual of one way this illustration could be adjusted for cover design.)