- On Writing - Stephen King. Brilliant book (and part autobiography too).
- The Oxford Concise Dictionary and Thesaurus. Like having the two features combined in one book. Never, ever rely on a computer spellchecker alone.
- Story - Robert McKee. Useful across all fiction writing types though especially aimed at scripts.
- How Not to Write a Novel - Sandra Newman and Howard Mittelmark. Hilarious writing guide. (Usually these books aren't funny at all but this one is brilliant both for its humour and advice).
- Wannabe a Writer?/Wannabe a Writer We've Heard Of? - Jane Wenham Jones. Very funny and full of really useful hints and tips.
As for listing what fiction books I wouldn't be without... well that would take me to Doomsday!