One of my favorite books for getting started is the "Millionaire Real Estate Investor" by Gary Keller.

PM me and I can send you a referral to my preferred lender (you do have to be recommended by a Keller Williams agent for this mortgage option).Since your strategy is to hold these properties over a long period of time, you can leverage your existing (paid-off) properties to fund the loan on the the new property, that way you are not coming out of pocket, and the rent will more than cover the refinance.

Not sure if you want to be independent or join a team, but may be worth checking out the Keller Williams in the area.  

One of my favorites is "The One Thing" by Gary Keller and Jay Papasan.

Speaking as a licensed realtor with Keller Williams, you generally will not see as much of a return on an ADU investment. 

@Ningxi Borak reach out to Robert O’Keefe of Keller Williams

Like Gary Keller says in "the Millionaire Real Estate Investor,"  Opportunities are always there in every market and in every time. 

I am also a realtor with Keller Williams (DRE 01105499) and I have access to the MLS. 

Invest In Real Estate and The Book On Real Estate Investing With Low And No Money Down by Brandon Turner.The Millionaire Real Estate Investor by Gary Keller, Sell Or Be Sold by Grant Cardone, really there are so many, check out BiggerPockets bookstore, google major investors on here.  

Reach out to Michael McManus at Keller Williams.