Numbering levels of a tree I find the need to use an explicit level numbering for a tree.
i.e. in the tree:

    A
   / \
  B   B
 /\   /\
C  C C  C


should I number the level C the 3rd and A as the first, or the opposite?
As going 'up' the tree is going towards A; should the numbering reflect that as well?
 If can, please supply an authorized CS reference.

Thanks!
There's no such thing as "an authorized CS reference", since nobody has the authority to declare how tree levels should be numbered.

The "height" of a vertex is its distance from the root (or distance plus one; see another question about the definition) so it probably makes sense to start at the top in most cases.  But you should use whichever numbering is most convenient in your situation.