The most important thing to remember when creating or publishing a book is that the cover is everything.
The first reason it must be professional is that conflicting to the saying 'don't judge a book by its cover' it doesn't really apply to actual books so the cover must be up to a high standard to make a sale, otherwise, the person who may otherwise be interested in making a purchase will simply move on. You can also get the information on booklet cover layout online.
Secondly, there is usually no way you can browse a book on the Internet, the only thing your potential customers will see is the cover if this isn't up to scratch these customers will just move on and look for a more professional looking book.
Furthermore, a professional cover means people are more likely to trust the things you say in the book and will be more likely to research around your existing products, increasing the chance of more sales considerably.
Another reason is if you have listed your cover on some of your existing websites and it is just a basic, boring cover they won't even think twice about clicking your link to purchase your book, they will simply disregard them and move on.
An eye-catching cover will also catch people’s eye who are just simply browsing online book stores, if they are just whizzing through pages and pages of books with simple covers then the cover of your book will seem to explode out at them from the screen, this means they will at least check out what your book is about, raising the chance of a potential sale.