Wael Hallaq, An Introduction to Islamic Law (Cambridge University Press, 2009), chapter 2, “The Law: How is it Found?”
Kecia Ali, Imam Shafi‘i: Scholar and Saint (Oneworld, 2011), chapter 3, “Legal Theory I: The Risala, Sunna and Hadith.”
al-Shāfiʿī, The Epistle on Legal Theory, trans. J. Lowry (New York University Press, 2013), Chapter on the Confirmation of Analogical Reasoning.
NB: This work is often referred to by scholars using its Arabic name: as the Risala fi usul al-fiqh or simply the Risala.
The following articles in the Enyclopaedia of Islam (available online via library catalog) may be useful: fiḳh, al-Ḳurʾān, Ḥadīth.