Multiple realities
(covers information from several alternate timelines)
An engineering notebook was a collection of notes on engineering data.
In 2375, during his last visit with Moogie, Rom brought home a Starfleet PADD that he was using as an engineering notebook. While getting up in the night to find something to eat, Zek found the PADD, which contained a schematic for a multidimensional transporter device, which Chief Miles O'Brien had given Rom to study. It was this schematic that gave the Grand Nagus the idea to travel to the mirror universe in search of new business opportunities for the Ferengi.
Zek explained all this to Rom and Quark when they were detained in the mirror universe. (DS9: "The Emperor's New Cloak")