Help on determining voltage differences (basic circuit theory)
You're entirely correct. \$V_{BC} = -9V\$

Re-arranging the schematic may make it more clear. Voltage difference from A to C is 12V - 9V = 3V because BAT2 is pointed in the same direction we're traveling from A to C.

The book probably just made a typo and meant \$V_{CB} = +9V\$. Note that typos are quite common in many textbooks I learned from. Be especially wary of first editions. They haven't had a bunch of students/professors sending in all the corrections yet in the first editions. You would hope by the third edition, someone would have cared enough to fix it.







simulate this circuit &ndash; Schematic created using CircuitLab