The Tale Spinner, by David Donaghe

Brandon Merry Weather is a young man with two passions in life: numbers and the written word. Brandon makes his living crunching numbers and unwinds by reading a good book. One morning when Brandon steps out of the front door of his Manhattan apartment building, he hears a clap of thunder and sees a flash of blue light. Brandon closes his eyes in fear and when he opens them, he finds himself in Greedy Gulch, the 1880s boomtown from the novel he is reading. The people of that world call him the new Tale Spinner and expect him to finish the tale because the author of the Mojave Kid Series died before finishing the work.