As far as landlording a small number of properties, most of the knowledge involved is how to be handy, or knowing trustworthy people who are handy. One piece of advice I would offer here is to charge slightly below-market rent to get better tenants, especially if you're an amateur.
And something a lot of people misunderstand is the relationship between appreciation and cash flow. They're inversely related because lots of appreciation attracts more buyers who drive down rental margins by bidding up purchase prices. Rapidly appreciating properties in a place like Seattle may have lower cash flow than you would anticipate given the demand, and properties with very little appreciation in a state like Ohio may have much better cash flow. But as I said, you should buy a local property and just accept the market conditions. If you want to invest in real estate elsewhere just invest in a REIT.