Charles Cardell was involved in Witchcraft at the same time as Gardner. He claimed to have been initiated as a 'Wiccen' by his mother and seems to have been involved with a group whose practices revolved around a God called 'Atho'. Initially Cardell appears to have been quite friendly with Gardner, but over the years animosity between them grew.
In 1964, shortly after Gerald's death, Cardell, under the pseudonymn 'Rex Nemorensis', published much of the 'Gardnerian' Book of Shadows in a small pamphlet entitled 'Witch'. The information is thought to have come into Cardell's hands via a woman called Olive Green (A.K.A. Olwen Armstrong Maddocks), who had received an initiation and the Book of Shadows from Gerald in the late 1950s.
It is also possible that Cardell may have obtained material from the Gardnerian Book of Shadows from Gardner himself as there is evidence that could suggest a mutual exchange of material, which would have been in keeping with both Gardner's and Cardell's characters.
You can read my article about Charles Cardell, Raymond Howard and the Head of Atho.
A recent article about some original Coven of Atho papers, along with a few images of them, held at the Norfold Record Office can be read here
For more information about Charles Cardell please check out his entry on my Other 1950s Witches page.
Cardell at the entrance to Dumbledene