If you agree to become a Goblin Engineer, then right-click on the Pledge of Secrecy and speak once more with Nixx Sprocketspring in Gadgetzan.
Goblin engineering is about practical uses for high profit and higher explosive power! Our schemata allow us to make powerful bombs and mighty weaponry; the manual you read covered this.
The manual also covered our ages old oath of secrecy on our schemata. Once you become a goblin engineer, that's that. There is no access to gnome engineering at all.
If you still want to become a goblin engineer, sign this document pledging life-long secrecy and speak with me again. Think about it carefully, [player].