Is it possible to create a spreadsheet where I type a Category Number in
Column C and the name of that category automatically shows in Column D?

Specifically, I have an inventory of books for our resource center. When
the Inventory was created, it was determined that both the category number
and description should be included in every entry. (I haven't worked with it
long enough to know how necessary this double-entry might be.) But it sure
would be nice not to have to type even the beginning of the category names!

Any ideas? Thanks in advance.