Having a third-party book about a product is an enormous marketing boost. Regardless of the size of your user base, a book gives users additional information about the product. A book also gives potential users a chance to evaluate the product and (hopefully) decide in favor of buying it.
Most importantly, a book adds mindshare: when people see the product’s name on a book in the bookstores, it validates you and your product as a contender in its field. Having a book about your product can mean substantial differences to sales.