A transition metal can be defined as an element that possesses an incomplete sub-level in one or more of its oxidization states. In the textbook I'm reading, it claims that zinc is not a transition metal because it has a full $d$-sub-level in all its oxidization states.
A quick google reveals that zinc has oxidization states $-2, 0, +1$, which means that zinc(with oxidization number $+1$) has an incomplete d-sub-level and is a transition metal.
What's going on here? Is my textbook incorrect?