Major influences? That's a snap: Edith Wharton, Joseph Conrad, R. L. Stevenson, Lee Child, Carl Hiassen, Rilke, Rumi, the Brontes, Dickens, Melville, Steinbeck, Fitzgerald. The list goes on and on. Contemporary novelists like Orhan Pamuk also belong in the pantheon. Classics like Crime and Punishment and Anna Karenina come to mind.
I like novellas, and write them myself, so seminal influences include Ethan Frome, The Great Gatsby, Frankenstein. If they have an element of adventure story to them, all the better: The 39 Steps, Kidnapped!, She.
I can't overlook short-story writers who have given me both pleasure and inspiration including Alice Munro, Andrea Barnett, David Gilbert. And of course old masters of the form like Gogol and Chekov.
A few writers have rised far above their genre like Japrisot's A Very Long Engagement, Lee Child's The Killing Floor, and Carl Hiassen's Skinny Dip.
As for nonfiction, I admire Tom Wolfe, Tracy Kidder, and John McPhee. Some wonderful memoir/biographers have stayed with me, particularly Richard Holmes' Footsteps, Mary Karr's The Liars' Club, and Slavomir Rawicz/s The Long Walk.
Many poets appeal to me, and have influenced me, foremost Rilke, Szymborska, and Rumi. I also like Billy Collins and some of the modern poets he's featured in his 360 collections.