This is great when there are sales, you can get the sale price on a book and not need to use a credit. There should be four "strips" or buttons you can press when you go to purchase a book — the top should be "Buy for 1 credit" (this one is typically orange, the other buttons a gray color), the next "Buy for $X.xx", "Add to cart" for the third, with the last being "More options."

One other note; it is good to check reviews. Some books are a condensed version and they aren't always properly marked but you'll typically see people complaining about it in the comments. Also, some of the readers are better than others, so you might want to see what people say about the reader/"voice actor" in the comments — and if in question make sure you listen to the sample to make sure you like or can put up with the book reader.