I have developed an interest in studying Pythagoras, mainly his idea of Number as the Arche. It seems that neither him nor his school left any writings, and what survives of it comes through Aristotle's and Plato's references in their respective pieces. However, according to Aristotle's Sources for Pythagorean Doctrine (J. A. Philip, 1963), Aristotle only ever makes two references to Pythagoras himself. So, where could one read about the Pythagorean school of thought and its cosmogony, in a way that's the farthest possible from second-hand?
Kenneth Sylvan Guthrie's Complete Pythagoras may be one place to start.
Patrick Roussel describes the collection as
The Complete Pythagoras is a compilation of two books. The first is entitled The Life of Pythagoras that contains the four biographies that have survived from antiquity....The second is entitled Pythagorean Library and is a complete collection of the surviving fragments from the Pythagoreans.
Guthrie, K. S. Complete Pythagoras. Retrieved on June 16, 2019 from Internet Archive at https://archive.org/details/TheCompletePythagoras