Fonts In Use is a remarkable blog that features and details the use of fonts (letterforms) in real world graphic designs in pretty much every category (e.g., advertising, branding, packaging, Web, etc.). Useful as that is, that it tells us what fonts are used in the designs makes it invaluable.
Frankly, the whole site is an excellent resource for design ideas and inspiration. What may be of particular interest to wordpreneurs, however, is its huge Books category. As of this writing, there are nearly 4,000 entries (yup, thousand) in this category alone.
If you still can’t find book cover design ideas and inspiration from this, keep that complaint to yourself, since no one will believe you.