Caught by Margaret Peterson Haddix (Lexile: 730; Reading Level: Grades 5-8) is the 5th book in the Missing series about children who have been kidnapped from their proper times in history. Jonah and Katherine have to help restore the children to their proper timelines.
In this book, Jonah and Katherine travel back to the early 1900s in Switzerland and Serbia to restore Lierserl (Albert Einstein's daughter) to her proper timeline. There is some difficulty for the kids as they try to understand where they are and what happened in the original time because they had to read some of Einstein's complicated notes. And, it appears, that Einstein was investigating time travel! Jonah and Katherine discovered that this was NOT part of the original timeline, but how could they stop it? And what if Einstein's wife knew they were there (even though they were invisible)?
I love reading this series, which is a cross between science fiction (time travel) and historical fiction (the times to which they travel). This series is one you should probably read in order, so I would start with Found. I am looking forward to book six: Risked, which was just published in September. Caught has 343 pages.
One of the reasons I became an elementary school librarian is so I can read children's books.