The haruspices were foreign specialists who never stopped being described as such. Roman writers called their body of learning the Etruscan discipline and divided it into three parts: books on entrails, books on lightning and its interpretation by region of the sky, and books on ritual procedure and the founding and bounding of things. Haruspices were retained by magistrates, consulted by the senate on prodigies, and organised in an order that survived into late antiquity, several centuries after the Etruscan language had ceased to be spoken.
Not one of those books survives, in Etruscan or in translation. What is known of the system comes from Roman authors describing it from the outside, often centuries later and often for their own literary purposes, and from objects. The most important object is a bronze model of a sheep's liver, found in the nineteenth century near Piacenza, whose upper surface is divided into a border of compartments and an inner field, with the name of a divine power inscribed in each. It is plainly a teaching or reference instrument for exactly the kind of reading the Roman sources describe.
What it cannot supply is the method. The liver names the regions and says nothing about how a mark in a given region was to be read, how the regions were weighted against each other, or what procedure turned a set of observations into an answer. There is no manual, and no scholar has been able to reconstruct one from the object; the atlas records that as an unknown rather than as a matter of scholarly disagreement, because there are no competing reconstructions on offer, only an absence where the method should be.