The Islam that reached the Turkic peoples of the steppe came through Persian-speaking cities, and its books were in Arabic and Persian. Ahmad Yasavi is the point at which it begins to be transmitted in Turkic, in verse, in a metre and idiom an audience on the steppe already used.
The Divan-i Hikmet, the book of wisdom, is a collection of short poems of instruction, repentance and mortality, addressed to ordinary people and built for recitation and memorisation. It is attributed to him and it is the foundation text of Turkic Sufi literature. The attribution needs care. Every surviving manuscript is centuries later than the twelfth century, the language of the collection as we have it belongs to a later stage of Turkic than his own would have been, and specialists treat the corpus as a tradition that accumulated around his name rather than as a book he wrote and left. That does not make it a forgery. It makes it a school.
His own training was orthodox and urban. He studied at Bukhara under Yusuf Hamadani, in the line of the Khwajagan masters from which the Naqshbandiyya also descends, and then returned to Yasi. The Yasavi practice is a vocal remembrance, the dhikr-i arra or sawing dhikr, named for the sound of the breath, and it is the loud counterpart to the silent method the Naqshbandis later fixed.