This may sound a bit hinky, but I find it a real struggle to write a novel unless I've got the correct music playing the background. Before I start on a new book I'll spend a ludicrously large amount of time searching for exactly the right soundtrack to my labours. Once I find it, the requisite music will then be played on a loop while I write. Doesn't matter if the book takes three weeks or three months, that music is all I'll listen to while I'm writing it - over and over again. In some cases that means I'll have heard the music hundreds of times before I'm done.
The music creates a mood, gives me a shortcut back into the story and the style of writing I've adopted. It blocks out any extraneous background noises. And it helps me focus on the task ahead. Sometimes the music itself can infect the speed at which I write. My third novel (an undistinguished Dredd effort called Silencer) was written with Michael Nyman's score for The Piano as my soundtrack. That mixes up slower, moody pieces with uptempo jigs. I found myself typing languidly in the slow sections and machine-gun fast while the jigs were playing. Strange but true.
I almost always opt for instrumental music while I'm writing, otherwise lyrics from the background start to infect the novel. The sole exception to this is if the soundtrack features singing in a foreign language. Being decidedly mono-lingual, people singing in German or Latin or Elvish makes no difference to me - their voices become just another instrument. Film soundtracks are a popular choice for me, although I will program out any tracks that could be distracting. I want smooth, seamless background music, not quirky tracks that call attention to themselves.
When I first started writing novels, I could remember the soundtrack for each of my tomes. John Barry's Dances with Wolves went with my first Dredd novel, while Philip Glass's Low Symphony matched the second. The score for Sneakers by James Horner was a good match for my first Doctor Who novel, Who Killed Kennedy. But I've written another dozen novel since then and subsequent soundtracks have begun to blend together.
Among my favourites are Han Zimmer's Backdraft, The Lost Prince by Adrian Johnston, Jon Brion's score for Magnolia, the Leon soundtrack, John Barry's The Last Valley and thomas Newman's The Shawshank Redemption. (His efforts on The Road to Perdition are great too, but - like a lot of movie composers - the differentiation between that and his earlier works becomes harder to distinguish over time. If you've heard one John Barry score from the last 20 years, it can feel like you've heard them all.)
My current project, Fiends of the Eastern Front: Twilight of the Dead, is being written to Michael Giacchino's score for the video game Medal of Honour: Underground. It's the first time I've chosen a video game soundtrack as my accompaniment, but the WWII vibe fits nicely with the book, especially now I've screened out the more overtly Parisian tracks.
Giacchino provides incidental music for the TV show Lost - now there's a soundtrack I wish they'd hurry up and release! In the meantime, my search goes on for fresh background sounds. I don't know what my next novel will be, but I'm already looking for the music to accompany it...