Magic Treehouse: Night of the Ninjas
Mary Pope Osborne
This is a great next book if your reader:
Would find the concept of time travel in different settings interesting.
Is just ready to read short chapter books.
Likes adventure and mystery around the world.
Would find a series with the same language, the same structure, and the same characters comforting.
If your reader enjoys tis series there are over 50 books in total - all with Annie and Jack solving time travel mysteries and a set of non-fiction companion books, called Fact Trackers.