Title and the description, I have found myself looking at that more than not.
Title is the bait, it has to be interesting, make you want to pick up the book or at least find out more about it.
Description is the hook, this is the ad, the little write up on the back of the book, on web pages, or in mags. It needs to tell you what makes this product different than others and why you will find it worth your money.
Content is the sinker.
Now people talk art, inside and out, but that is flash and while it helps sell some products I don't trust it as much. Plus this is the day of the web, look at the top of these pages, banners and marquee messages, art is limited.
Don't get me wrong, there are companies and people that I will look at just because of their names but they have earned my respect from prior products but they are only a member of a team, I really don't know if the editor, proof-reader, layout person, and so many others are the same one that put out product X or not. Sometimes it is the people behind the product make it a great one or kill it.