Plutarch, De iside et osiride. (Best translation is by Robert Graves)
Vera Vanderlip The Four Greek Hymns of Isidorus and the Cult of Isis (Listed by Amazon as out of print)
Malcolm Drew Donalson: The Cult of Isis in the Roman Empire
My previous post quoted from the first two books. The Vanderlip is a secondary source, but contains the primary sources complete.
Sorry, sorry, in my previous post I accidentally deleted a chunk of text, and conflated two references.
Please amend thus:
Plutarch, De iside et osiride (Best translation by J G Griffiths)
Apuleius, The Golden Ass (Best translation by Robert Graves)