Although the book has In Oz in the title, Dorothy and the Wizard spend very little time actually in Oz. But dont let this put you off. The underground lands that they pass through are every bit as exciting and magical as the different lands actually in Oz. The ending (how they escape the underground world) is a bit weak, but the imaginative countries that they pass through and the adventures they have in each more than make up for this.
Dorothy and the Wizard in Oz is a book that you will want to start reading again as soon as you finish, but dont. Go on to the next Oz book and then the next. While I believe that this was the best of the 14 original books in the series, they are all wonderful and I would recommend that everyone read the entire series from beginning to end.
This is a great read; I would suggest it to non-Ozophiles so that the MGM movie can be challenged, and the true Oz can be appreciated in its majesty of fantasy, humor, horror, and splendor.