To find statistics about the value of direct marketing, use Passport GMID. Do a keyword search for "direct selling" using the search box at the top right. Use filters on the left side to find reports about direct selling in specific countries (including the US), or view the Global Briefing about Corporate Direct Selling Strategies.
Search the Library Catalog for books about direct marketing. For example, see Direct Marketing in Practice by Brian Thomas and Matthew Housen, which is available as an e-book.