Ethan's mom is the most important character we never actually see. A Civil War historian from Savannah, Georgia, she died five months before the book began. But before she passed away, she and Ethan were close—he could tell her anything. This means that without her, he's pretty much lost.
Lila and her son had quite a bit in common. For one thing, Ethan's mom loved the library: "This is my church, Ethan. This is how we keep the Sabbath holy in our family" (9.14.172). Now we know where Ethan gets his healthy appreciation for literature. We miss this gal already, and we never even met her.
And unbeknownst to Ethan, his mom and Marian (the librarian) were working on a biography of Genevieve and Ethan Carter. To top it off, Mom led a double life as the Caster Librarian—that means that she knew about Casters and magic. See what we mean about her having a lot in common with her son?
Lila's ghost (or what Ethan assumes is her ghost) pops up at opportune moments to guide Ethan from beyond the grave. All of the hints and tips Ethan gets from this unseen force remind him of his mother: he finds clues in her favorite recipe book, and the door to the study, where she kept her books, magically unlocks. But we never see her.
When Ethan sees Genevieve's ghost, Aunt Del observes, "Seeing the dead is not one of the gifts of a Caster […] One can only see the dead if the dead so will it" (12.07.99). So why doesn't Lila will herself to be seen?
P.S. We'd like to mention that her name sounds like that of another famous dead literary mother—Lily Evans. Lily Evans Potter, that is. What else do these two ladies have in common?