If you're looking to get started in real estate, look at a crowd funding solution like RealtyMogul. It works similar to LendingClub - you commit as little as $5,000 towards a property. When the property is fully funded, you become an owner, and will receive your share of the earnings and appreciation in the property.  Check out RealtyMogul to learn more. 

I wish I had more time to put into real estate. Given the run up since 2012, I may even be interested in selling my condo that I currently rent out. I need to get it appraised to really see what it’s worth, but I think conservatively it’s gone up ~50%, although rent is probably only up ~10% or so. I am bullish on rents going up in the future… mostly in line with inflation, or perhaps even slightly faster due to constricted credit and personal income growth which should provide a solid supply of renters. At this point, I just don’t want to manage the property. I’ll probably look into a property manager as my time is likely worth turning it into a nearly passive investment.
I have not. While I am intrigued with the possibility of making online income, it seems to be less passive then how I want to spend my time. Regarding your blog / site, you have done quite well for yourself. However, you have to keep pumping out content or your site would eventually go out of business. That sounds like more of a commitment then I would want. Regarding your book sales, it is probably relatively passive now, but certainly was not when you were writing the book. Now if you love it, great. Just not for me.
In equity valuation, residual income represents an economic earnings stream and valuation method for estimating the intrinsic value of a company's common stock. The residual income valuation model values a company as the sum of book value and the present value of expected future residual income. Residual income attempts to measure economic profit, which is the profit remaining after the deduction of opportunity costs for all sources of capital.