From the first page:
Spirit Magic is the practice of animism and ancestor worship. Spirit magicians call upon spirits which manifest their powers on behalf of their summoner or binder. These beings have many
names... dryads, nymphs, diseases, ghosts, nightmares, elementals, ancestors; yet in truth, are all sentient fragments of magical energy from which everything is derived. A spirit does not teach or use magic, rather they are magic.
Spirits flow between the mundane plane and the spirit plane, eternally transferring the power of magic to the mundane world and back again.
Spirit magicians interact with these beings, utilising the spirit’s magic to augment or protect themselves. Such sentient immaterial beings are powerful allies, being capable of independent action and manifesting their powers for unlimited periods of time.