Book XX of the Atharvaveda has 143 hymns and is unusual in the canon: the great majority of its hymns are direct re-borrowings from the Rig-Veda, particularly Soma-Indra hymns from RV books VIII and IX. The Atharvan tradition is doing canon-politics — its inclusion among the four Vedas was historically the latest and most contested, and demonstrating that the Atharvaveda can authoritatively cite the Rig-Veda was part of how the tradition argued its scriptural status. For most of Book XX you can read the commentary on the parallel hymn in [Rig Veda](rig-veda) and the substance carries across. The chapter-by-chapter content of Book XX up through hymn 126 is largely review material; the genuinely Atharvan-distinctive content begins at hymn 127 with the Kuntāpa cluster.