Not read it, though I've flicked through it in bookshops:
The Vesuvius Club by Mark 'League Of Gentlemen' Gatiss is a graphic murder-mystery/spy novel* full of Beardsley-esque Edwardian dandyism, decadence and transsexual weirdness. Not set in London, but it seems to chime in rather well with
Decadence/Aestheticism/Symbolist/Pre-Raphaelites/opium dens/etc etc.
*i.e. a graphic novel about mysterious murders, not graphic murders - although they may well be that, too
Edit: which of course brings to mind
The Picture Of Dorian Gray, an obvious one for this thread, no? Or were you more after modern writers writing about that epoch? Hell, I guess you could probably do a lot worse than read a bunch of Dickens (not that I have, friends of mine who've read him tend to describe it as unspeakably dull, though I assume it would be good if you want authentic descriptions of Victorian London).