"The Avesta (/əˈvɛstə/) is the primary collection of religious texts of Zoroastrianism, composed in Avestan language.

The Avesta texts fall into several different categories, arranged either by dialect, or by usage. The principal text in the liturgical group is the Yasna, which takes its name from the Yasna ceremony, Zoroastrianism's primary act of worship, and at which the Yasna text is recited. The most important portion of the Yasna texts are the five Gathas, consisting of seventeen hymns attributed to Zoroaster himself. These hymns, together with five other short Old Avestan texts that are also part of the Yasna, are in the Old (or 'Gathic') Avestan language. The remainder of the Yasna's texts are in Younger Avestan, which is not only from a later stage of the language, but also from a different geographic region."

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Information about the author of the book "Avesta, The Bible of Zoroaster", available below, is provided here. This may help readers to better evaluate the contents of the book.

"Abraham Valentine Williams Jackson, L.H.D., Ph.D., LL.D. (February 9, 1862 – August 8, 1937) was an American specialist on Indo-European languages.

He was born in New York City on February 9, 1862. He graduated from Columbia University in 1883. He was a fellow in letters there from 1883 to 1886, and an instructor in Anglo-Saxon and the Iranian languages from 1887 to 1890. After study at the University of Halle from 1887 to 1889 he became an adjunct professor of English language and literature. In 1895, he was appointed public lecturer and also appointed to the newly founded professorship of Indo-Iranian languages at Columbia University, where he remained until 1935.

He was well known as a lecturer on English literature and the Orient. In 1901, during a visit to India and Ceylon, he received special attention from the Parsees, who presented to Columbia a valuable collection of Zoroastrian manuscripts in recognition of the instruction there given by him in their ancient texts. In 1903 he made a second journey to the Orient, this time visiting Iran. He also visited Central Asia sometime before 1918.

Jackson's grammar of Avestan, the language used in the Zoroastrian scriptures, is still considered to be the seminal work on the topic. Jackson was one of the directors of the American Oriental Society.

He died on August 8, 1937."

           (from Wikipedia)

     Avesta, The Bible of Zoroaster