Help get this topic noticed by sharing it on Twitter, Facebook, or email.
I'm sorry for the delay in getting back to you.
I passed this to the editor of the book and he said:
There are a couple of ways to determine which is negative and which is positive. The book directs readers to trace through the circuit while comparing it to the schematic. The schematic shows which side is positive and which is negative. But you can also find negative leads by tracing the wire from the negative terminal and following it to all the wires connected to it as shown in the attached diagram.
I hope that helps,
O'Reilly Book Support