I really feel like the colors in this book cover work really well. The designer has used shades and tints to allow the speech bubble in the center to pop out. The black bar across the bottom also helps the colors stand out.
The inside spread of my book was tough to get it all just right. I used the same font from my title in the cover along with Minion Pro for the inside spreads. The serif and san serif fonts make a really good combo.