The first step to designing an effective book cover is to familiarize yourself with the content of the book. Whether you're designing a cover for a nonfiction or fiction book doesn't matter. Giving the book at least a cursory read is important. The cover of the book should reflect in some way what the book is about. The cover should serve to entice readers to want to get to know the contents of the book further.
Always capture the essence of the book. For a work of fiction, you'll want to use a main character or a recognizable scene from the book for your book's cover photo or illustration. It's always a surprising revelation when a reader reaches a certain scene or becomes familiar with a character and they can turn back to the cover of the book to get a visual representation of the printed page. If the book is nonfiction, use an illustration that emphasizes the goal of the book. If it's instructional, work that message into the illustration or cover text.
Cover balance means controlling the amount of text versus graphics on your book cover, using colors that work well together and avoiding graphics that detract from the overall presentation. The aim is to make a book cover stand out as a whole upon initial viewing, then allow the graphic and text elements to work on an individual basis. You want the overall cover to grab the reader and deliver a message, then the individual elements to entice the reader to study the cover further. This means arranging the elements of the book's cover so they promote easy eye flow. Collages, for instance, are usually not effective.
Pay particular attention to the font style and color you use when designing a book cover. A fancy script font might look good, but if the text is too hard to read, most potential book buyers will move on. Use a bold, clean font for the book's title and the same for the author's name. You want to avoid too much text on the front cover of a book; any text that you do use should be unobtrusive.