I’ve recently completed the proofing of a book that required American English, and that footnotes etc. followed The Chicago Manual of Style. Luckily, I’ve written in American English for many years, so not much was new to me, but I did buy the latest edition of the CMOS to be on the safe side, as well as Merriam-Webster’s Collegiate Dictionary (the US equivalent of the OED).
I have to say, the CMOS is amazing! For around the first 25% of the book I found myself consulting it about every five minutes – and it has an answer for everything. The only thing I was left a little unclear about was the reference format for some websites, so I just did the best I could to be consistent. Although the subject matter of the book was a little heavy going, I found proofing it surprising enjoyable as it gave my little grey cells a bit of exercise!