Most Voyager Field Workers don't have a History wiz like Jeffrey Jones at their side, but they've no need to worry. The Guidebook, loaded with facts and important dates, is there to help them give history a shove where its needed.
The Guidebook is a history textbook of facts designed specifically for Voyagers. It resembles a leather-bound bible and is engraved with the 'V' symbol and each Voyager's name in the lower right-hand corner. Phineas and Jeffrey fumble through history without it and do pretty well...for now! However, the Guidebook is the necessary tool that all Voyagers must carry on their assignments.
The Guidebook provides a Voyager with vital information – names, places, and dates – in order to bring history back to a green-light state. We never see the inside of the Guidebook on the series, so we'll have to take their word for it. Voyager Olivia Dunn is the only one who actually used the Guidebook on her missions. When Bogg thought she was going to retire he tried to take it off her, but she decided to stay a Voyager.
After the Phineas Bogg's trial, Councilor Garth proudly presented Bogg and Jeffrey with their brand new Guidebook. However, the boys were too happy at the outcome. Jeffrey was meant to be a Voyager and could stay at Bogg's side. Jeffrey jumps into Bogg's arms and they disappear to resume their missions, leaving Garth and Voyager Susan confused.
By the end of the series, Bogg and Jeffrey still don't have their Guidebook.
It would've been great to start off Season 2 with one, as the historical adventures become more complex. Jeffrey stopped schooling in 7th grade and he hasn't joined the Academy yet, so there's no way he could know every part of history.
There exists at VHQ a full code-book. According to the Guidebook Replica, the 'real' Guidebook automatically updates itself. It's printed by the same mysterious people that manufacture the Omni and is actually written in their language, but each Voyager can read it in his own language, which tends to support the hypothesis that the omni has a translation function. I would posit that there is a "Planet Voyager," and that there is also a race of people indigenous to that planet who have solved the problem of time-travel. They produce the Omni and the Guidebook and pluck people out of the time-stream to become Voyagers.
In "Billy and Bully," Jeffrey tells Bogg the Code says "No romance while the Omni's red." There's no such actual Code, but there is one that says no Voyager is to become romantically involved with another Voyager. It may apply mainly to field workers, though it doesn't specify in the Guidebook. So much for anything serious ever developing between Phineas and Olivia, and quite possibly, the reason why Phineas only admired Susan from a distance.
*The Guidebook Replica information was provided by fan Jake Crepeau.