Affiliate programs are the current hot thing in monetising websites. The basics are that retailers like Amazon will pay you a percentage if you send someone to their website to buy something. The percentage ranges from around 3% with sites like Amazon all the way up to 60% or even higher for information products sold by other people looking to make passive income. 

As an struggling young Engineer (back in the Carter era) I bought anything I could renovate then rent to justify paying the 18% interest. I never took vacations but worked on my properties all in the pursuit of passive income. I drove junk for many years & many months I just got by on credit cards. My friends & colleagues were amused by my ‘stupidity’ but most are still working to make enough for retirement.
I agree mostly with the real estate advice. I’m looking for ways to take advantage of the condo I own to get up the rent from ~$0.90/ft to the $1.2-1.5/ft that seems more like the range in the same area. I’d have to put in a bit of capital (probably 10k on the low end for just the basics up to 40k if I wanted to remodel the kitchen and 2 bathrooms up to par with the area), so the return is likely there if those upgrades warrant $1.30/ft (given the unit is larger than most 2br/2ba in the area).
Passive income is a great tool for building retirement income faster, paying off debts, and, ultimately, retiring early and comfortably. If you have passive income during your retirement years, you potentially could live as well as you did during your peak earning years. Passive income is money you earn without doing actual labor. Often times, it comes from investments, such as in rental properties, stocks, bonds, annuities, and other investments.
The books are pleasantly organized, customers served, teams managed, etc. The problem (in my mind) is that this is hugely time intensive and easily capped. Income = Time x Value, where Time is a finite resource limited to 24 hours a day (or, more realistically, 12).The way most people address this is to change the second variable. They get a law degree, MBA, or PhD, making their time worth more money. This works swell… but it’s still limited. At some point, you’re that lawyer making $550/hr, but your income potential is capped, as willingness to pay more is at it’s highest, and no new time can be created. What to do?
Creating passive income is the key to retiring, especially retiring early. If you want to retire early with the stock market you will have to save a huge amount of money, because that money has to last so long and retirement calculators depend on you using your investment to fund your life. With passive income you will have income coming in as long as you own the investment, without every eating away at the principle invested. Creating passive income may take more work than investing in the stock market, but I think it is well worth the extra effort!