Understanding how current is 0A in secondary circuit of this potentiometer - misprint?
Ah, one of the joys of being an old fart is seeing instruments I've used displayed as museum pieces. Such as a Biddle Portable Potentiometer and a similar device equipped with an optical galvo. Similar to this one. 



Anyway, the voltage across the divider chain bucks the unknown voltage. When it is exactly equal then no current flows through M2 (typically a very sensitive galvonometer). Your analysis should yield a voltage of the same polarity as the unknown, the book is correct. Maybe you have an issue with the 0V reference, but without your work it's hard to guess.