You are correct. There is a book near where you found the mirror, between or beside some packages/crates.
BTW, don't tell either healer that you have cured their patient until you have cured both of them.
Check your quest log on this one; it can contain additional hints on what to do next.Elrath, the Plague and the Poisoned Harvest
Is Tingalf still around? Lt Robin should have had him arrested.Marcus-Tingalf-Trevor-Quest