For MMGM, I am recommending an old classic: The Secret Garden by Frances Hodgson Burnett.
(Note: The cover above is of the Barnes & Noble Classics edition, which I read as an e-book.)
Today, I'm going to skip the description and make up my own: After her parents died of cholera, rude, grouchy Mary is made to live with her uncle (who is a widower) in his large house known as Misselthwaite Manor. While living there, she gradually becomes happier as she befriends people (such as Martha, a servant in the home, and Dickon, Martha's brother who is 12) and discovers secrets of Misselthwaite Manor, such as the garden of Mary's late aunt, which became locked after Mary's uncle buried the key in grief.
This book was first fully published in 1911, and it has aged remarkably well, becoming one of my favorite books of all time! One thing I loved about this book is how Mary's change of personality is handled gradually, making her seem more realistic. She slowly warms up to the other characters and her new life. Another great part of the book is the setting (specifically, the location.) It is well-described and makes you feel as if you were living there yourself. The story also has many characters with varying personalities, ranging from those who have it in them to be kind, such as Dickon, and those who have to find it for themselves, such as Mary. All in all, The Secret Garden is a masterpiece that is just as good now as it was 105 years ago!