Is this circuit from a textbook wrong?
For true TTL tri-state operation the output must be able to pull up and down. The circuit in your book can only pull up, which is not compatible with TTL logic.

Here is a real tri-state TTL (inverting) buffer:- 







simulate this circuit &ndash; Schematic created using CircuitLab

When Enable is high Q3 is turned on, which turns off Q5 via D1, and Q4 and Q6 via Q1B. Q6 provides the essential pull down when not tri-stated.