Jan Van Quirm said:
Drum's not 'in' the staff at all - that's a duracell or a piggy-bank that's rechargeable from free-ranging magic and from it's owner/charge. Drum
IS a parasite -
on the apple tree. Granny also visits the tree
which is thickly covered in misteltoe - that's where Drum is, not in the tree itself. Also lots of druidic connotations there which also is fairly compatible with witch magic of course
Terry certainly wanted Esk to get a cartload of magic to use, but she can only soak it up gradually as she doesn't have the experience to use it effectively - yet.
While I agree with a good bit of what you are saying here about the staff being a rechargeable battery from the free-ranging magic & about Drum Billet having set post death spells on the staff to protect itself and the new owner until the owner can be made a wizard him/herself, but I'm afraid I must disagree about the mistletoe.
Drum Billet has taken up a non-reincarnation temporary (apparently) existence in the apple tree--the one which, in the intervening years since Esk's birth has grown much taller than the other apple trees, and which she can climb and be alone and apparently eat apples while her brothers find the tree essentially unclimbable. It is also covered with lots of mistletoe, which makes it appear green even in the winter (as described on the day when the children are sent to check on Granny).
When Esk flees the house because the raven (with Granny inside) trying to get in frightens her, she flees to the woods where she is threatened by wolves and saved by the staff before Granny can get back into her body and come looking. Granny, having put Esk to bed, borrows an owl and goes to Drum Billet
WHO IS NOW THE APPLE TREE.
The two of them have a long discussion about wizard and witch power. At first, Drum/Apple tree tries to push Granny/owl off but she refuses to go and he says,
"Bully me then, just because I'm a tree. Typical woman."
Later (the Tree/Wizard having explained to Granny the "theoretical basis of witchcraft" ,and stating that he is
"Not a wizard any more just a tree," Granny flies back to her home and Esk. At that point Granny is still determined that Esk will be a Witch not a Wizard. Not until Esk loses herself in the Eagle and the Staff and Granny have to co-operate to find her and untangle her mind from that of the bird, does Granny give in to the realization that she must take Esk to the Wizard school where she can be trained in the use in wizard magic. It is then that she gives to staff to Esk, and it protects itself and her until she becomes a full wizard on her own.
Thus, Esk is the first (and probably the only) witch/wizard Terry ever created. This seems to be a set of ideas which he will take apart and use in future books quite effectively. But, in my opinion, if Terry were to live to 100 years of quality writing life, we still would not have a book featuring the further adventures of Esk and Simon.