There are 13 books in Euclid's Elements.
'Elements' was written by Euclid.
need a simple explanation of Euclids theory.
The thirteenth book in Euclids Elements has no specific title, but covers the Pythagorean proofs of Theaetetus (c. 417-369 bc) and the construction of the five regular Platonic solids (pyramid, cube, octahedron, dodecahedron, icosahedron) inside a given sphere.
His mathematical books called Elements. His work on geometry, particularly his famous axioms.
The Elements which are 13 volumes