When I mention a book, an e-book, a software product, or a product sold by somebody else they are linked to an affiliate code. I’ve written these books, bought a book or item and liked it, or I wish I could buy it.
These “affiliate programs” pay a referral fee. Amazon has an affiliate program. Apple’s iBooks store has one, and so do B&N and Kobo. Any book on this Web site that links to Amazon, iBooks, B&N, or Kobo includes my affiliate code and I will get paid for the referral. If you don’t want that to happen then open those stores and search for the items to purchase.