Late 90s mystery w/ Isaac Newton hunting a counterfeiter

I read this in 1998 or 1999 and I'm pretty sure it was new at the time. It was a mystery featuring Isaac Newton during his time as Master of the Mint tracking down a counterfeiter. (Much of this actually happened, but this was definitely fiction.) I have the feeling that the author was more of a mainstream writer than a mystery writer. I was thinking maybe Robert Harris, but it isn't him.

Dark Matter by Philip Kerr? (It's a couple of years later than 1999 but seems to fit otherwise.)

This is part of Neal Stephenson's Baroque Cycle trilogy. The hunt for the counterfeiter comes to a head in The System of the World. The first novel Quicksilver was published in 1999.

This is not the book you're looking for, but in addition, you might like Newton's Cannon by J. Gregory Keyes.

None of these, alas. I've read Newton's Cannon, never read any Stephenson and the book was a standalone. The Kerr comes closest, but it's simply too late. I definitely read the book between September of 97 and June of 99, based on where I was living when I read it.

Thinking about it, the story may have been narrated by a young assistant or at least had a tight 3rd person POV from that perspective. Hardback, darkish cover in the blue/gray range, title and author in large lettering, only artwork something in a circle as was so typical for cover art at the time.

I read Newton and the Counterfeiter last year, but it wasn't fiction

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https://en.wikipedia.org/wiki/Isaac_Newton_in_popular_culture#Books_featuring_Ne... doesn't add anything to the previous suggestions.