"Бух" is simply reduced joky form of "бухгалтер". It seams that "booker" is the same for "bookkeeper"

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Note added at 22 mins (2017-06-19 20:14:40 GMT)
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Or even just "Book"

One possible way of conveying the diminutive ''Бух'' [Главбух] is by transforming the word "chief" into a diminutive, while omitting ''accountant'' altogether.

I read this as an emphasis of the importance of the guy. You can pick the term that best suits the rest of the context.
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"The Man" is a slang phrase that may refer to the government or to some other authority in a position of power. In addition to this derogatory connotation, it may also serve as a term of respect and praise.

https://en.wikipedia.org/wiki/The_Man

-ster is a mocking suffix for a surname in US EN. E.g., The Hilzster vs. the Trumpster. Evokes a state of being, and a hamster. Possibly from The Hulkster, for World Wrestling Federation champion Hulk Hogan.

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Note added at 57 mins (2017-06-19 20:49:31 GMT)
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http://www.washingtonexaminer.com/the-trumpster-mark-hamill-...

Also evokes Dumpster, trade name for a large trash bin. https://en.wikipedia.org/wiki/Dumpster

Cf. dumpster fire https://www.google.com/search?q=dumpster&oq=dumpster&aqs=chr...

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Note added at 1 hr (2017-06-19 21:34:06 GMT)
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-nik also is a humorous suffix calque, like "nudnik" in US EN from Russian Yiddish.

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Note added at 1 hr (2017-06-19 21:34:33 GMT)
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Thus, Buchnik.

I think the name should depend on whether you call the man's position chief bookkeeper or chief accountant. If he is a bookkeeper, than Bookie may work since it clearly carries across the origin of the nickname - something Buch or Buchster doesn't quite do, because really, home many of your readers do you expect to know German? (Well, maybe in Europe, but in the US it would be a lost cause. Unless good Ashkenazi Jewish folks from Brooklyn come to the rescue:)). That said, I am fully aware of the new connotations, totally absent from the original, that Bookie brings in. Still I think it is somewhat better and more fun than Booker here (Booker Prize, anyone)?

Now, if you call him an accountant, methink you could probably go with County - and yes, again, I am fully aware of the new can of worms I am opening here. I am just thinking out loud.

I don't like Bean Counter at all. It's perfectly fine for referring to someone in the 3rd person, but as a nickname it's way too long, and it's probably what everybody and his brother would expect you to call the guy, which kind of makes it a cliche. And where's the fun in that?

accountant