A book and its cover look best when they are designed together. The interior design can pick up elements from the cover, and the cover can use typefaces and type styles found on the pages of the book. One test of a good design is the experience of opening the book for the first time and seeing that the inside reflects the mood and style of the cover.
Many authors want a print edition plus an ebook edition. Your designer can design a cover and interior with this in mind.