No book of the Etruscan discipline survives in any language. The Piacenza liver assigns divine names to regions of the organ and says nothing about method, and the Roman authors who describe haruspicy were writing from outside the tradition, often centuries later, and were not attempting to transmit a procedure. Nobody has proposed and defended a reconstruction of the method, so what is missing is evidence rather than agreement.
What would resolve this An Etruscan haruspical text of any length would settle it, and a bilingual would settle it decisively. Failing that, a systematic comparison of the Piacenza liver against the far better documented Mesopotamian liver models and their accompanying omen compendia would at least establish whether the two systems share a method, which would constrain the possibilities.
OpenEtruscology, Etruscan epigraphy, AssyriologyNancy Thomson de Grummond and Erika Simon (eds.), The Religion of the Etruscans