The back cover lists it as "The Locked Door" by William Clarke Wontner, but since they misspelled the artist's name (it's incorrectly listed as William Clark Wonter on the book) I don't trust the citation. The link to the book on Amazon is below, any help would be great.
http://www.amazon.com/Sarah-Canaan-Trilogy-Marek-Halter/dp/1400052785/ref=cm_lmf_img_2_rsrsrs0
http://www.amazon.com/Sarah-Canaan-Trilogy-Marek-Halter/dp/1400052785/ref=cm_lmf_img_2_rsrsrs0