You can get the car door magnets from Vista Print pretty cheap. If you want one that is bigger, just design two or three of them to go side by side for your tailgate.
The auto insurance shouldn't be a factor, but it's okay to ask. An advertisement for your book is no different than any other bumper sticker with a website. Commercial auto is for transporting goods or services for a fee. Delivering pizza's or newspapers with your personal car would fall into this so you should have commercial insurance. Or where the use of said car is by non-owned drivers and employees. Also it's used to cover real property loss on hauled equipment so if you crash your truck while hauling two riding lawn mowers for your lawn mowing business, the mowers are covered. In other words it provides wider and higher levels of liability that personal auto policies do not.