Various works with no known author.
Also see anonymous works from the early Church fathers.
The Cloud of Unknowing(115 pages)
A Book of Contemplation Which Is Called The Cloud Of Unknowing, In Which A Soul Is Oned With God
Theologica Germanica(51 pages)
This work was discovered and published in 1516 by Martin Luther, who said of it that “Next to the Bible and St. Augustine, no book has ever come into my hands from which I have learnt more of God and Christ, and man and all things that are.” It has since appealed to Christians of all persuasions.