What was the first gamebook you read (that wasn't your own)?
Metal City Mayhem (Sonic series)
What is your favourite gamebook?
Creature of Havoc or Moonrunner, can't decide which.
Captif d'Yvoire (T&T) and Warbringer (Way of the Tiger) also stand out, but they suffer from belonging to mundane series, IMHO.
What gamebooks/interactive fiction would you recommend to a newcomer to the genre?
Midnight Rogue, Vault of the Vampire, Creature of Havoc. In exactly that order.
Summarise what a gamebook is to a newcomer in 100 characters or fewer.
A novel in which the decisions of the reader or random events have an impact on the plot.
Why are gamebooks great compared to games or books?
Please define the terms "game" and "book".
Where did you come up with your ideas for gamebook game systems (in this case additional rules for FF)?
Actually, I don't think I would add additional rules if I ever wrote one. Additional rules deviate from standard rules, and could be stressful for the player to implement if they are either too complicated or are implemented too frequently. They could also be difficult to implement for a computer-coded version, thus limiting their development (I converted Lair of the Troglodytes as a C programming exercise, but stopped at converting ones with more complicated rules, since those rules were incompatible with my central subroutines dealing with stats and combat).
When it comes to writing a gamebook, what's the most important thing that you do?
Make sure it would be worth reading. Not necessarily fun - educative or inspirational would do too.