Green’s Dictionary of Slang

debt; thus in the mercer’s book, the state of being in debt.

1591		Greene Farewell to Folly To Gent. Stud. sig. A4: Such wags as [...] haue marched in the Mercers booke to please their Mistris eye with their brauerie.
1601		Jonson Poetaster III i: Faith sir, your mercer’s booke Will tell you with more patience, then I can.
1675		Character of a Town-gallant in C. Hindley Old Bk Collector’s Misc. 4: Thence he posted to one of the Inns of Court, but [...] never read six Lines in Littleton, for he loved a Placket better than a Moot-case, and was more in his Mercer’s Books than in Cokes.