This is a fun way to disguise a lot of neat math ideas in a story, that even has lots of pictures. Reading it gives a kid the feeling that they are being let in on weird secrets that their teachers probably don't even know.
Top Secret: A Handbook of Codes, Ciphers, and Secret Writing
This is a pretty light introduction to secret codes. It spends most of the time on ciphers, but also discusses secret inks, creating a secret hiding box, and a few other "spy" topics. There is a little hint of math when it talks about letter frequencies used to crack codes, but not much. Overall a fun book.
This is a great book for 4th or 5th graders who like to read biographies (such as Who Was Walt Disney, etc). Math is usually pretty boring in school, so this book is great for kids to realize that there are fun stories behind it. There's almost no math here, just easy to digest stories about Archimedes, Newton, Gauss, etc.
This one is great for the whole family at dinner or bedtime, especially if you have younger kids. Each fun, well illustrated topic has four parts. First, a story that is good reading practice for 4th/5th graders. Next, three questions of increasing difficulty that are aimed roughly at 5 year olds, 7 year olds, and then 9 year olds.
Elon Musk is an incredible person (although very controversial). He's not only brought electric cars to the masses, but he started an incredibly successful space company that will take us to Mars, designed a new transportation system in his spare time, and is also working on allowing our brains to communicate directly with computers so that we don't get taken over by AI. Most biographies about him are not appropriate for kids, so I thought it was great that there's a young reader's edition I can share.