Alvin Plantinga 
Warrant and Proper Function [PDF ebook] 

In this companion volume to Warrant: The Current Debate, Alvin Plantinga develops an original approach to the question of epistemic warrant; that is what turns true belief into knowledge. He argues that what is crucial to warrant is the proper functioning of one’s cognitive faculties in the right kind of cognitive environment. Although this book is in some sense a sequel to its companion volume, the arguments do not presuppose those of the first book and it stands alone as a stimulating contribution to epistemology.