“Living with Klinefelter Syndrome (47,XXY), Trisomy X (47,XXX) and 47,XYY” by Virgina Isaacs Cover is a very good book, which explains Triple-X on page 107 ff. in a very good and compact way.
“Elizabeth and her Extra X” by Arlie Colvin explains Triple-X very good to children
Diana Divera has written 5 books about Triple-X and her experiences. You can ask her to get a free ebook copy in English or Dutch.
Sarah O. M. Schmidt blogs on Instagram in German and English
Three Baby Kisses blogs on Instagram in English