The sale of knowledge through a book, e-book, physical format, or online course or training program is a popular passive income stream that doesn’t require a lot of capital upfront. The Amazon KDP platform also makes self-publishing a book very easy. No publishing contract is required. Your manuscript can be uploaded easily by creating an Amazon Publishers account. You will receive a 70% refund if your book is priced at least $3.99. You must also have a professionally designed cover.