I have been working my way through a translation of Diwan Malkuta Laita (Scroll of Exalted Kingship) that Jorunn Jacobsen Buckley published. The scroll is a liturgical guide to the consecration of new priests among the Mandaeans and is full of tightly-packed nuggets of their particular Gnostic cosmo-ontology. Considering it as a well-preserved piece of a dialogue with other Gnostic practices from antiquity has been very rewarding.
(I’m not saying it is just a well-preserved bit of dialogue, for it is part of a living Mandaean practice, too. I am, however, outside of that living practice, so for me it is most readily accessible as evidence rather than as testimony.)