Let me make life easier for you: have you read Possession, by A.S. Byatt? 
 If yes, read and enjoy the pleasant sensation of deja vu. 
 If no, go read Possession; you'll have a better time. 
 In The Forgotten Garden's defense, it is a good story. Told from the perspective of several of the main characters, it navigates the reader carefully through three generations of repression, secret keeping, and guilt. It also brings into pointed relief the reality that, even after we die, the fallout from our decisions has the potential to be felt for years afterwards. 
 One of the things that's interesting to me about this book is the presence of guilt. It's one of the main emotions of the book:(view spoiler)[ you see it in Eliza, for not saving her brother, for being whole while Rose is sick. Nell's father lives in a constant state of guilt for not turning Nell into the authorities until the day he dies, and imagine what would have happened to her if he had. Nell stays in Australia to care for her granddaughter because she feels guilty for being an indifferent mother to her own daughter. (hide spoiler)] It's almost like the whole book is a study of what happens when guilt motivated the human heart-obviously, nothing good. 
 What I disliked about this novel is pretty trifling. It was predictable-that's a mean criticism, though, because true originality is hard to find, and really, the book is so well written that it seems petty to mention that there was only really one direction the story could ever go in, given the voices that the author chose to use. I also would have wished to hear more of Nathaniel's opinion. He feels like half a character that came in nearly at the last minute, yet what we see of him is likable and easy. (view spoiler)[ Frankly, I don't understand why he loves his wife; she's fairly awful. I was hoping he would make a run for it with Eliza, honestly. They both would have been happier, and probably have lived. (hide spoiler)] 
 All in all, it was a good book. If you like mysteries without the added bonus of a ghost or two, go ahead and read The Forgotten Garden.