Almost everything a reader knows about Roman religion arrives through literature written when the old cult was already being self-consciously curated. That is the central problem with the evidence and it is worth naming first.
Virgil, Ovid and Livy are magnificent and they are late. The Aeneid is a poem of the Augustan settlement, the Fasti is a calendar poem written by a man in trouble with the regime that commissioned the mood it flatters, and Livy is reconstructing early Rome centuries after the fact from annalistic material he could not verify. All three are indispensable and none of them is a record of what an archaic Roman thought he was doing at an altar.
The one writer who might have closed that gap is largely lost. Varro's Antiquitates rerum divinarum was an enormous systematic treatment of Roman cult, and it survives mainly in quotation by Christian authors who cited it in order to attack it. What we have of the most important book on Roman religion is therefore a hostile selection from it.
Against that stands the epigraphic and calendrical evidence, which is unglamorous and much closer to practice: dedications, priestly records, the surviving fasti carved on stone, the Arval Brethren's acts. These do not tell stories. They tell you who sacrificed what, to whom, on which day, and they are the reason it is possible to say that Roman religion was orthopraxy before it was mythology.