Second paragraph of third chapter:

Theodore hadn't stopped apologising for the last mile, even though Yaz kept insisting it was unnecessary – if she hadn't clicked that, outside the protection of the TARDIS, any update to K9's systems would overwrite the data, then he couldn't possibly be expected to know.

A good Thirteenth Doctor novel by the experienced Jac Rayner, taking us into the world of L. Frank Baum's Oz, and neatly written on the assumption that the average reader will be half-familiar with the 1939 film but maybe less so with the 1900 book. K9 and a guest character, Theodore, bulk out the usual TARDIS crew (although two had been written out months before this book was published last summer); Graham, Yaz and Ryan recapitulate Dorothy's companions on her journey; and we get a bonus invocation of The Mind Robber (which I'll come to shortly). The worst thing about the book (and it is not all that bad) is that the question of Who Is Behind It All is thoroughly spoilered by the cover. Otherwise it's decently in the traditions of both Who and Oz, and I think would be accessible to fans of the one who don't know the other.