I can only remember bits and pieces of the book. A girl finds out she can see magical creatures after going to her grandparents house. In the middle of the book she finds a journal (or it's her journal) that she buries by a tree. She finds secret passages that lead to different parts of the house. One passage leads to a hidden nursery where I think the grandma hid her daughter.
Could this be the Fablehaven series written by Brandon Mull?
The series begins as 13-year-old Kendra and 11-year-old Seth Sorenson are traveling to their Grandpa and Grandma Sorenson's house while their parents are away on a 17-day cruise. When they get there, they also meet Dale, the groundskeeper, and Lena, the housekeeper. Grandma Sorenson is "mysteriously" missing. Grandpa Sorenson does not tell Kendra and Seth about Fablehaven being a secret preserve for magical creatures at first, but instead sets up a rather complex puzzle involving six keys and a locked journal for Kendra to solve. Once Kendra unlocks the mostly blank journal, she discovers the words "Drink the milk." She and Seth drink the magical milk Dale sets outside in their yard every morning, opening their eyes to a whole new, mystical world full of the magical beings of Fablehaven. Then Kendra and Seth must face challenges such as defeating an evil witch and a powerful demon, defending the preserve from an evil society, stopping a plague that changes creatures of light into creatures of darkness, accidentally turning Lena into a naiad, and, ultimately, protecting the world from a horde of imprisoned demons.
I had previously suggested this book as an answer to Series where a girl joins a magic creature guarding society.