Wow, where to start!?!
I tend to shoot urban exploration and steam railways, but I've got a strong interest in photo-journalism. As such I've been influenced by the British photo-journalists of the 1960's who were known as the Young Meteors - the likes of Don McCullin, Graham Finlayson, Ian Thorpe, etc. Not so much in their subject matter, but in their pictorial approach to documentary photography, and that's something I've tried to embody in my own work.
I've also got a huge admiration for the work of Cartier Bresson, his blend of compositon and timing captivates me.
I also like Martin Parr (or at least some of his work, his most recent stuff is self indulgent nonsense IMO) for his humour and sharp observation. Many people can't stand his work though.
In my own genres, I admire the work of Railway photographers such as Colin Gifford, who pretty much led the way with the 'new approach' to railway photography in the 1960's, capturing the last days of steam in Britain in it's grim but magnificent reality in a non-traditional, creative way. O.Winston Link is an American railway photographer from the 1950's whose work has to be seen to be believed - you'd be amazed even if you've got no interest in railways at the scale of his lighting set ups.
Finally, the urban exploration scene is a fairly new genre, so there's not many wel known photographers let alone mainstream books to study. The exception is Henk van Rensbergen, a Dutch airline pilot and recreational trespasser whose website got me into urbex a few years back. He's got some good individual images, but his genius is in the selection of the images as a set to illustrate an explore.
I'm sure there's some others, but these are the ones who've influenced or inspired me the most!
)