The truth is that we possess but a trifling portion of a very much larger Avesta, if we are to believe native tradition, carrying us back to the Sassanian period, which tells of a larger Avesta in twenty-one books called nasks or nosks, as to the names of which we have several more or less detailed accounts, particularly in the Pahlavi Dinkard (9th century A.D.) and in the Rivayats.
We are told that of a number of nasks only a small portion was found to be extant " after Alexander."
Of all the nasks one only, the nineteenth, has come down on us intact - the Vendidad.
It would be rash summarily to dismiss this old tradition of the twenty-one nasks as pure invention.
The number twenty-one points, indeed, to an artificial arrangement of the material; for twenty-one is a sacred number, and the most sacred prayer of the Parsees, the so-called Ahuno Vairyo (Honovar) contains twenty-one words; and it is also true that in the enumeration of the nasks we miss the names of the books we know, like the Yasna and the Yashts.
Further, the statements of the Dinkard leave on us a very distinct impression that the author actually had before him the text of the nasks, or at all events of a large part of them: for he expressly states that the eleventh nask was entirely lost, so that he is unable to give the slightest account of its contents.
But, even granting that a certain obscurity still hangs undispelled over the problem of the old Avesta, with its twenty-one nasks, we may well believe the Parsees themselves, when they affirm that their sacred literature has passed through successive stages of decay, the last of which is represented by the present Avesta.
(309-380) the nasks were brought into complete order, and the new redaction of the Avesta reached its definitive conclusion.
West, Contents of the Nasks, S.