The Dasam Granth is a separate volume from the Guru Granth Sahib Ji, of about 1,428 pages, containing compositions attributed to Guru Gobind Singh Ji. It was compiled after his death, with Bhai Mani Singh Ji's name attached to one recension.
It contains the Jaap Sahib, the Akal Ustat, the Bachitra Natak, the Chandi compositions, the Zafarnama, the Charitropakhyan and other material. Several of these are in daily use: Jaap Sahib, Tav Prasad Savaiye and Chaupai Sahib are in the nitnem, and the opening of the Ardas is from Chandi di Var.
Its authorship is one of the longest and sharpest disputes in Sikh scholarship. Some hold the whole to be the Guru's; some hold only parts to be; some hold the Charitropakhyan in particular, a large collection of tales of deception including many of a sexual character, to be court material by other poets.
There is no agreed critical edition and the manuscript recensions differ.
The dispute is not academic. It determines which texts are recited, and there have been formal takht interventions and public confrontations over it in recent decades.
The language is largely Braj rather than the Punjabi of the Guru Granth Sahib Ji, with Persian in the Zafarnama, and that difference is itself part of the argument about who wrote what.
Parts of it are recited daily by every amritdhari Sikh and other parts are the subject of a dispute about authorship that has run since the nineteenth century and is not resolved.
The manuscript history is the crux. The earliest recensions differ, the compilation was made after the Guru's death, and no critical edition acceptable to all sides has been produced.