@Jason Malabute I went from a fourplex straight to a 20 unit. It is all really the same thing until you get to 100 doors. At that point, it gets a lot easier because you have on site management. The only trick part of this is when you hit "land lord hell" as one investor called it. Once you get 30-40 doors you will always have a vacancy and you will always have a maintenance issue. That is when you really start to appreciate a property manager if you have been self managing up till then! You need a lot of maintenance people to help you, and leasing becomes a regular part of the business. 

The other part no books (that I know of) talk about is whether to vertically integrate in a local market (i.e. build out your own management infrastructure and maintenance team) by acquiring many smaller deals, or trading up into larger buildings (100 units plus). I have been meeting more and more local operators here in Chicago who have vertically integrated, but they aren't taking the time to document this path for any other investors. It seems like an interesting strategy as many of them own several hundred units which obviously provides plenty of wealth and cash flow.