A book cover needs to work on many levels. It has to look good, look current, stand out from the crowd, and make a strong connection to what’s inside. Oh, and it has to sell the book.
For interior layouts, the challenge is to find the right balance of text, illustration, and white space. You need to set the text in a solid, readable, versatile font, with a reader-friendly size and weight. Design principles are more straightforward for pages than for covers, but both require a combination of artistic and technical skills. It also helps to be a bit of a font geek.
See my cover designs of The Secret Garden for three very different approaches to a classic title.