I'm looking for a story where the main characters, fairly sure that they were some kids and a grandfather, who often traveled into books. (This happened in other books in the series, which I haven't read.) In this one, they traveled to Oz. They landed with Dorothy, where they realized that their extra weight threw the house off and it didn't kill the Wicked Witch. They then spent the rest of the book trying to fix it.
This was in English, I have no idea when it came out. I think that story was aimed at ~11/12 year olds.