A good source of folklore and surviving pagan practices are church documents that describes practices evident in pre-christian Roman and Greek sources, archeological evidence, and early modern folkloric sources.

This book is the longest description I have read of this type of source. It discusses to what extend these texts can be read as actual evidence of existing practices, or just per verbatim repetitions of existing church texts. It never really concludes anything but mostly agrees that these really are evidence of existing practices.

It is split in multiple sections for different types of practices.