Who was Euclid?
Nobody knows much about Euclid's life anymore - it is all forgotten. We only know that he worked at the University of Alexandria, in Egypt, for a while. There are no pictures of him. We can't even be sure he existed; Euclid could be a made-up name for a committee of mathematicians all working together.
Either way, Euclid (or the people who called themselves Euclid) must have lived around 300 BC, in the Hellenistic period. He (or they) seems to have studied at Plato's Academy in Athens, where he learned some of the mathematics that is in his books. He probably knew Aristotle there.
Like Anaxagoras before him, Euclid (YOU-klid) wanted to prove that things were true by using logic and reason. We still have copies of Euclid's books today, and they begin with basic definitions of a point and a line and shapes, and then go on to use geometry to prove, for instance, that all right angles are equal, that you can draw a straight line between any two points, and that two things which are both equal to the same thing are also equal to each other.
Euclid's later books teach more advanced math, including how triangles and circles work (Euclid proved that the area of a circle had to be πr-squared), irrational numbers, and three-dimensional geometry. Euclid's interest in proofs is probably related to the interest of Jain mathematicians in India in logic and proofs about the same time, though we don't know enough to say which came first.
Euclid is famous because his books were so easy to understand that they were used as the main math book in all schools in Europe, West Asia, and America for two thousand years, until the 20th century. The geometry book Anne of Green Gables hates so much is Euclid. There are still mathematicians working today who began studying geometry from Euclid's books.
Greek and Roman Science, by Don Nardo (1998). Nardo has written a lot of good books about the ancient world for kids; this one is no exception.
Ancient Science: 40 Time-Traveling, World-Exploring, History-Making Activities , by Jim Wiese (2003). Activities, as the title says - how to make your own sundial, and so on. The author is a science teacher.
Early Greek Science: Thales to Aristotle, by Geoffrey Lloyd (1974).
History of Greek Mathematics: From Aristarchus to Diophantus, by Thomas L. Heath (1921, reprinted 1981). A lot of Euclid, but also describes who the other major Greek mathematicians were and what they did.
Episodes from the Early History of Mathematics, by Asger Aaboe (1997).