The Spanish, Roman and Portuguese inquisitions of the early modern era relied heavily on the methods and theories developed by the medieval inquisitors. The Dominicans formed the 'pool' from which the 13th century Papacy drew its first inquisitors and they embraced the task due to their reforming zeal, their mendicant work within communities, and their trained theological-intellectual acumen. Most famous of these early inquisitors is Bernardo Gui who wrote what became the text book for inquisitional practices, the Practica Inquisitionis Heretice Pravitatis completed in the 1320's. David Burr has translated and published online portions of the text. You'll see fairly quickly a pattern of these manuals: they are thoroughly analytic works dealing with matters of fine theological examination of heresy, in the case of the above link Gui is tackling the beliefs (as he sees them) of the Beguines. Gui's manual also lays out a procedural how-to of examination and inquiry, and collates Papal documents which authorize inquisition.

Janet Shirley has translated the manual:

• Bernardus Guidonis, The Inquisitor’s Guide: A Medieval Manual on Heretics, translated and edited by Janet Shirley. (Ravenhall Books, 2006)

A recent good (academic) book on the development of inquisitorial thinking and writing in the middle ages is:

• Christine Caldwell Ames, Righteous Persecution: Inquisition, Dominicans, and Christianity in the Middle Ages (University of Pennsylvania Press, 2011)

If you want a good, very readable overview of the history of all inquisition, the standard starting place is:

• Edward Peters, Inquisition (University of California Press, 1989)