It doesn't look like Mr. Book was one of the actual 'Ethiopian Serenaders', but it does seem that it had also become a generic term for blackface minstrel.

If he were my ancestor, I'd be researching the dickens out of that one!

There were two 1st KY Cavs - Butler's and Helm's. I don't see a James or J Book or anything similar in either one, but you might search Footnote.com - it's a paysite but they have a 7-day free trial.