The Seer of Shadows by Avi, published by HarperCollins, 2008.
This historical fiction starts off with a photographer trying to perpetrate a fraud by putting ghost images into portraits. But, in an ironic twist, real ghosts begin to show up in the photos taken by his apprentice, Horace, the novel's first person narrarator. I won't give it all away, but there's more to it than just the ghost wandering through while the picture is snapped.
I was attracted to the novel for the similarities to my current work in progress, a mystery with a ghost revealed in photographs.
Avi's story moves along at a good pace and the plot and characters are solid. Set in New York in 1872, the language is sometimes a little formal, but nothing insurmoutable.