Do you have any books or resources out there that he can dive into to help him hopefully avoids some of the common pitfalls?

I was just reading Mark's book "The Tax & Legal Playbook" this morning and it specifically had a "critical warning" (page 31&32 in Chapter 3).

I would suggest starting with reading the IRC book all the way through multiple times.

Of course, with all things, the answers to all these matters will depend on the circumstances.California tends to have more laws on the books and requirements and restrictions that it can be a good idea to form a CA LLC for out of state property so that you as a CA resident are covered, and to try to have your contracts fall under the purview of CA courts.

Read some books. 

., managing bookings or tenant interactions), which could subject it to self-employment tax.This post does not create a CPA-Client relationship.

Here's a https://www.irs.gov/businesses/cost-segregation-audit-technique-guide-chapter-4-principal-elements-of-a-quality-cost-segregation-study-and-report to the IRS website noting specific items that are included in the cost segregation study report. hmm, that website is a 404 error.  

I've read BiggerPockets' Real Estate Investing book and am particularly interested in the house hacking strategy, as I'd like to move out of my current living situation.Our main dilemma is choosing between a single-family home (renting out one room to a close friend) or a multifamily property like a duplex or triplex.

I recommend you read some books on how to manage investments well.

@Dennis KnappJ Scotts book on estimating rehab costs is a great start. https://store.biggerpockets.com/products/the-book-on-estimat...Here is a "general" rule of thumb for my market in Jacksonville, FL.