God Created the Integers, by Stephen Hawking
Hawking has compiled an historical anthology of 31 well-chosen works from 17 mathematicians. These are the works of major contribution that students seldom (never?) see in text books. We see in text books only how to use and what to do with the results, not how the result itself was developed in the first place. Hawking has reproduced the original proofs here, along with a brief biography of each contributor. It's a large book, but it was lots of fun to read.
I hadn't appreciated that Euclid is thought to be a mere cataloger of existing work. There isn't much supporting explanation in the biography. I would think that the style and characteristics of the proofs bearing his name ought to be sufficient evidence to lend credence to original work. Perhaps elsewhere Euclid himself described his role.