Especially today, when books are sold mostly online, you should avoid white backgrounds. White backgrounds disappear on a page, especially when shown as a small thumbnail. Use a color, a texture, or a background illustration instead.
Colors are important because colors create emotional reactions. The colors you pick should be connected to the story and mood. For example, a book best suited for the beach should consider incorporating cool blues and jade greens to give a tranquil feeling. Dramatic books should consider using black, bold red, and similarly dramatic colors to underscore the mood of the story.