It was generally used by Sophie instead of Brian because she preferred to. However Brian used it on occasion. (The Crying Game, Smotherly Love)It may have been used by Throg if he existed. (Full Metal Sophie)
As noted above, the Guidebook showed infomation on alien cultures. It required that the research to be typed in. Then it would display a holographic image of the subject. The infomation shown was sometimes of considerable complexity. (Beauty and the Breast: Part I, Wrongs of Praise)
The Guidebook could interface with the house/ship and do many alerations. (The Tail of the Knitted Map)These incuded enlarging rooms (Brian of the Brownies) and swapping around rooms(The Tail of the Knitted Map). Also it can add a inbuilt cleaner and delete the recycle bin. Aware what damage it could cause Lucy Barker considered it dangerous. (Brian of the Brownies)
The Guidebook has various computer viruses contained within it. One of the most notable was the EPV, or Electrical Personality Virus, which was used by Lucy to gain extra revision time. ('Goodbye Mr Flips)
In Series Eight the Guidebook is updated to an artificial intelligence named Guido, where it is revealed that Brian and Sophie were originally meant to update the out of date guidebook. Due to compromising their mission their memories are wiped and their mission started again. Instead of using the Guidebook to type out information required Guido relays the last known information, with Brian and Sophie then updating it later in the day.