The books in this series are all great! If you try to research many of the inventors and scientists in the series based on articles on the internet, you'll find that they are frequently written on a level well above what an elementary school student can understand. This is particularly true of Luis Alvarez. These books are well-written, include fun illustrations, pictures, and cartoons, and are easy to understand for children. I would say they are on a 3rd-5th grade level, and they are about the right length to cover the basics without becoming tiresome for kids. Well done!