The Anubis Gates by Tim Powers
Brendan Doyle is a twentieth-century English professor who travels back to 1810 London to attend a lecture given by English romantic poet Samuel Taylor Coleridge. This is a London filled with deformed clowns, organised beggar societies, insane homunculi and magic.
When he is kidnapped by gypsies and consequently misses his return trip to 1983, the mild-mannered Doyle is forced to become a street-smart con man, escape artist, and swordsman in order to survive in the dark and treacherous London underworld. He defies bullets, black magic, murderous beggars, freezing waters, imprisonment in mutant-infested dungeons, poisoning, and even a plunge back to 1684.
Coleridge himself and poet Lord Byron make appearances in the novel, which also features a poor tinkerer who creates genetic monsters and a werewolf that inhabits others' bodies when his latest becomes too hairy.