A cover usually gives the title the greatest prominence, followed by a subtitle or series marker and then the author name. The exact balance depends on genre, audience and author recognition. Strong contrast and readable type help the important information survive when the cover is reduced to thumbnail size.
Review the design both large and small, in color and grayscale if appropriate. If every element demands attention, the central message becomes unclear. A professional cover designer can create hierarchy through scale, placement, spacing and restrained use of color.