A good billboard is never more than a few words—the theory is that if it's something quick to read and easy to remember, it will come to mind naturally when people need it. The front cover of a book is the same; we want something fast, simple, and aesthetically appealing. The back cover copy is more complicated. It needs to be longer than a mere phrase, but just as eye-catching.
The front cover and the back cover are a team that sells the book. Without a good front cover, no one will pick the book up. Without a good back cover, anyone who picked it up will put it down instantly. A bland, emotionless set of sentences will repel as fast as a clever cover attracts. A good back cover is like the smell of fresh baked cookies or chocolate cake: irresistible. The copy needs to appeal to the reader, to draw them in, to say, "This book is written by someone like you for someone like you about something that you care about. You will benefit from reading this book." And just like that, it's in the shopping cart.
If you can do one well, you can learn to do the other well. Make your front cover like a billboard—and your back cover like the smell coming from a pizza shop.