Indeed, in terms of Wagner, literature certainly follows in the wake of music--no Wagner, no Baudelaire, no Symbolist Poetry, (no film music?) etc etc. Oh yes, re: film: no Grand Opera, no Cecil B. de Mille....

I was thinking more in terms of how contemporary writers have attempted to use music as a metaphor or theme that runs, fugue-like, through a narrative - my favourites being 'Music & Silence' by Rose Tremaine (of course, the book is about much more than lute music!) and 'An Equal Music' by Vikram Seth, where Schubert's Trout Quintet and Bach's Art of Fugue are recurring motifs. Writing about music in a non-analytical way, which makes it accessible to non-musicians, is very difficult to pull off successfully - I know, I've been trying to do it for the last 5 years! What Seth does so well, in my view, is create a sense of the player's 'involvement' in the music, and the way a certain piece, motif or phrase can pull you in.

I agree with Frances about the importance of hindsight here. And certainly to think of Debussy, eg, solely in terms of 'impressionism' is to miss a great deal in his music that has nothing to do with Monet et al.

Gavin asked:

"But the only two composers who have had a significant influence on other art forms have been Wagner and Cage. Why should that be?"

The short answer would be: they're two who have strayed most deliberately beyond the boundaries of the purely musical, getting involved in theatre, literature, philosophy, etc. (I'm reading a theatre studies-cum-architecture PhD at the moment for which Wagner is a great theoretical touchstone: but this is almost entirely related to the concept of the Gesamtkunstwerk and his architectural designs at Bayreuth that derive from this; there's not a mention of a single note of music.)

I'd also add Beethoven to Gavin's short list: the whole concept of the heroic, struggling artist derives from him.

Thank you, Tim, for reminding us about Beethoven, the greatest radical of them all! One only has to look at the Op 27/2 to see a composer poised on the brink of change, someone deeply and inextricably bound up in the prevailing political, social, literary and artistic landscape of his time. :-)

Cage and Wagner were both as important as theoreticians as they were as composers, although I don't know how may non-musicians (or musicans either come to think of it) have actually read their writings. I think it is also significant that both dabbled in other media, but not very well. My German wife cringes at Wagner's lyrics. She's sick to death of my recording of John Cage's diary too, although that may just be overexposure.
Beethoven on the other hand...well, it is amazing how influential he was outside of music given that he only wrote one opera, the poets he set to music were usually at least as famous as he was, and (unlike Cage) he wasn't much of a collaborator. His influence must just be down to genius, right?

I don't know about Wagner's writings, but as to Cage I'd say he was possibly even better read outside of music than inside (where it seems acceptable to pontificate loudly, abusively and lengthily without hearing a note or reading a word of his work). Certainly when I was studying and living with various visual artists at Goldsmiths they were all aware of his writings, even if they didn't know much about the music itself.

The Beethoven thing I would say is maybe as much to do with deafness, Heiligenstadt, the 'Dearly Beloved' as anything in the music. Maybe. At least, there was something about Beethoven's life, works or the combination of the two that established the paradigm for the 'great romantic artist' that we apply unthinkingly to almost anyone these days. It's not so much a question of influence, or anything that he even had any control over; more that he established something about the relationship between the artist and his/her society that has we now take for granted. (Scott Burnham to thread!)

Yes, Beethoven was a genius - but I'm not sure Beethoven would have considered himself a 'great romantic artist' - any more than Wordsworth, Byron or Blake, who were his artistic contemporaries - but I do think these people were working at a time of great change, politically and socially, and were sufficiently interested in what was going on around them to allow it to seep into their work, either consciously or subconsciously. I wonder, also, whether Beethoven simply had more artistic/creative freedom, because he was not in the thrall of a regular patron or employer (unlike Mozart et al, before him)?

These days, it seems that all artists are required to make some justification for their work, to 'explain' it, to highlight their influences, or what motivates them - no longer pure 'art for art's sake'.

It is curious though that there has never been any pressure on artists to restrict themselves to a particular medium. Musicians tend to be most productive and influential in the sphere of music because that is what they are best at, not because they are culturally restricted to the medium of sound.
Wagner was influential outside of music because he did not want his music restricted by the incompetency of other librettists, stage designers, architects etc. And whatever he said about Gesamtkustwerk, the music always came first. In a way, Cage matters more, because his work completely transcends, or rather ignores, the accepted distinctions between artforms and between media.
Hildegard is another interesting example of a musician who was influential outside of music...

Just going back to Beethoven, if his influence outside of music was based on his persona rather than his music, then could we not say the same about some fictional composers? What was the influence of Leverkühn and Vinteuil, on later generations of composers and artists, fictional or otherwise?