Jacqueline Rayner's Winner Takes All is one of the first New Doctor Who Adventures novels and features the Doctor and Rose as played by Christopher Eccleston and Billie Piper. It's also one of the funniest, yet it also has some very dark moments. It borrows the plot device "What if the video-game you're playing is actually real?" used by Terry Pratchett in Only You Can Save Mankind and in the movie The Last Starfighter where a video game called "Death to Mantodeans" isn't nearly as harmless as it seems. On another world, an alien war is raging: the Quevvils (who look like giant porcupines)need to find a new means of attacking the ruthless Mantodeans (who look like giant praying mantises). The Quevvils search the galaxy for a cunning, warlike, but gullible ally, and find the ideal soldiers on Earth. Rose and the Doctor finds themselves caught up in the game, literally, as they're taken to the home planet of the two alien races. Rose finds herself participating in the game, with her nervous system wired up to a control device which is linked to a game controller, which the Doctor is forced to use to ensure that Rose wins the game, and destroys the Mantodeans. In case he feels inclined to refuse to play, the various people who've supposedly won a free holiday (but have instead been transported to the alien planet) are threatened, including teenager Robert, whose single mother is also somewhere inside the "game".
Interestingly, of the handful of New Doctor Who Adventures books I've read featuring the Ninth Doctor, I think Rayner's book does the best job of depicting him. Winner Takes All is also available from Amazon.com.