On S.J. Day's website in the FAQs section, question #1 plainly states that The Marked series was never planned as a trilogy. And over in the Media Kit section the press release for Eve of Darkness says in the first paragraph:
Beloved author S.J. Day's action-packed Eve of Darkness (A Tor Urban Fantasy Mass Market Original, $6.99; On Sale April 28, 2009) launches the consecutive month releases of the first three books in the new Marked series.
I guess someone didn't do their research. :(