For book/ebook authors, publishers, & self-publishers
Okay, here's my two cents, an author platform is the foundation an author creates to sell his/her books. For example, their website, Facebook Page, Twitter, Pinterest, Instagram, Google+, email list, newsletter, along with the various Facebook groups and other writing oriented organizations that they belong to. The message varies from book to book but the "foundation" remains the same. You can use your foundation to promote any of your books if you do it properly.
Cheryl Callighan, PAA, MVA
The Author's Assistants