This is a series and the first book is all about a young city boy, Henry Yorks. A wall crumples apart one day when he was at his aunt and uncle’s house. Henry and his 3 cousins try to unlock all 100 doors but find only 99 doors. The last door to be opened is in his grandpa’s room which has been sealed shut for many years ever since Henry’s grandpa passed away.
The second book is Dandelion Fire which is what I am currently reading now.