The alphabet was made for scripture, and for most of its history the surviving evidence is scriptural, because parchment books were kept and everything else was not. The exception is Novgorod.
Excavations there, and at other northern sites, have recovered more than a thousand documents written on strips of birch bark, scratched with a stylus rather than inked. They are not literature. They are debts, instructions, complaints, lists, land disputes, marriage negotiations, a boy's school exercises, notes between neighbours. They are written by and to merchants, artisans, women and children as well as clergy, and the earliest of them are close to the beginning of the written tradition in the region.
Their importance for the alphabet is that they show it working outside the church. A script devised for liturgy was, within a few generations, the ordinary tool of people conducting their own affairs, and the level of lay literacy they imply is considerably higher than the book record on its own would suggest.
They also record the language rather than the norm. A scribe copying a Gospel writes what the tradition requires; a man writing to his brother about a debt writes what he says, spelling as he hears it. The birchbark documents therefore preserve the local speech of the period, including sound changes the liturgical books conceal, and they are among the best evidence there is for what was actually happening to the reduced vowels while the books went on writing them.