The first Who book my daughter read was Who was Queen Elizabeth when she was at 3rd grade. After reading, for a long time, she loved taking about Queen Elizabeth to people. She learnt a lot about Britain history, Britain and Spain war from the book and from the discussion with her father.
Ever since then, she loves reading Who books. Whichever Who she wanted, we bought for her. By 5th grade, she had 33 Who was Who books on her bookshelves. She read Who was Mark Twain, Charles Darwin, Pablo Picasso and Marco Polo etc..
These are simplified version of Who books, less than 100 pages long. But they provide a great wide knowledge to kids. They can learn history, science and other countries in a very fun way. I love reading them too on the bed.