I guess I didn't miss much by not reading this book as a child. I don't really understand why it became a classic. It starts out interestingly enough with a very gothic setting. A little British girl named Mary survives a cholera epidemic in India and is sent to Yorkshire to live with her distant relatives. The author gives a vivid description of the beauty of the moors and the mysterious mansion that the girl goes to live in. The only other interesting part is really when Mary discovers the boy who she hears crying in the mansion and when she discovers the secret garden. Everything else beyond that (which is most of the book) isn't all that interesting. The author spends many pages explaining how miraculous and magic fresh air is for healing and fattening up the crying boy and the girl who escaped the cholera epidemic in India.
The bits that get old after a while: Oh, look, it's a garden! Look, I can run and play! I'm not a cripple after all! Look at the pretty birds! The garden is alive! Now I have an appetite! Isn't it a magical miracle that I'm having fun playing outside?
I just wasn't really impressed. If you're going to write a novel in a gothic setting, you at least need a small ghost or a mysterious disappearance or something.