There are 13 books. Book 13 is about regular solids.
Regular solids
compositions
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.
There are 13 books in Euclid's Elements. The title of Book XIII is " Regular solids."
There are 13 books. Book 13 is about regular solids.
There are 13. Book 13 is about regular solids.
There are 13 different books in "Euclid's Elements". There is not a specific name for the 13th book but it is about Pythagoreans.
Euclid's Elements is a set of thirteen mini-books and sometimes found as one whole book. The thirteenth book it titled Regular Solids.
so it stands out and is noticeable. The title is what usually draws the reader to the book.
When you refer to the title of a book in a paper or article, you write the book title in italics.
Elements
Elements