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Unemployed 2014 grad until recently, just got an offer at a Big4 in a major market working in a niche subset of international tax. Basically I'm going to help advise American companies on dealing with tax systems in foreign countries.

Problem is, I know NOTHING about tax, or business, or accounting. I got the job because I know a few people at the firm, got along with the people I interviewed with, and did a lot of work in law school in a practice area thats conceptually related to international tax, but is much more focused on litigation and administrative law. As fortunate as I feel to have a job now, I really don't want to look like a fool when I start.

I start in about a month.

To what degree should I prepare?

Besides just dry review books, what sources are out there which would give me an overview of topics like commercial paper, international business transactions, international tax, etc? Any free online courses on these subjects?

Any help would be appreciated!!

From someone who graduated without a job, took 2 classes in law school that I remembered nothing from before starting in Fed Tax at accounting firm (which I then used to lateral to M&A tax), don't even worry about it. The people there will know you're a JD; there are different expectations for accounting majors/CPAs and they'll all know it. Just work hard to learn, there is nothing you will know before starting. Even if you had taken an international tax class in law school, it probably wouldn't even help you with the job.
I was in the same position/line of thinking, and I always hated the "they'll teach you everything you need to know" but it's true and it's just a learn on the job type of thing.
Don't sweat it, you're seriously good to go. Enjoy the freedom until you start!