I don't really know that there's much written about it. Lawyers tend to keep business models close to their chest and it was very much a learn by doing kind of thing.

When I took the job there was certainly a period of "mind blown" while I learned more and more about the industry but I had the perfect position to learn the industry.

For online advertising, all of the core skills about keyword bidding and targeting apply. Terrestrial marketing is vastly more effective in the legal space though and while more expensive (TV), billboard and radio/podcast pricing has come down significantly.

Really it's all about coming up with your own effective price model and making sure you can measure the effectiveness of your campaigns. Most law firms are vastly overspending compared to what they actually need to but they're spending in places that bring in so much money that they can literally throw money away at this. If you're data driven you can make a fortune here and there's tons of existing firms (and a small but growing group of marketing consultancies) to partner with if needed.

Having technical skills and a JD will make people want to work/partner with you.