Buy a small business: A local small business, like a car wash or a laundromat, is a great way to put money down on a money-making venture. Automate it so you don't have to be on the premises unless you're collecting money. Go into a local business with your eyes wide open - study the books, especially on income and expenses, and examine water and utility bills if your venture will be open 24 hours.

Non-fiction e-books that educate your potential audience on specific topics like finance, online marketing, and business are going to make you more money than fiction books. Of course, there are always exceptions and you could write the next Harry Potter book, but if you want to create some residual income opportunities quickly, I would suggest you go for what sells first!
I have only dabbled in drop-shipping before when I had an eCommerce platform 6 years ago or so. I think it is something that you could do on the side, but you would want to do in depth research on the industry you want to get into before setting up shop. It may be a little less passive up front, but over time you could take your hands off the wheel.