keys and variables

I'm trying to understand when and how to use keys.

What I want is to a) remove all explicit xpath expressions from the 
core of my stylesheets so as to allow one to use them with different 
schemas (let's say TEI for bibliographic representation, vs. MODS), and 
b) make it as fast as possible.
So let's take a minimal document:

<bibs>
  <book ID="1">
    <title-main>First Title</title-main>
    <author>John Doe</author>
  </book>
  <book ID="2">
    <title-main>Second Title</title-main>
    <author>Jane Smith</author>
  </book>
</bibs>

Basically, I want the main xslt files to have expressions that are 
generic: like maybe $title and $creator in this case.  Ultimately there 
may be twenty or so structures so defined.  To use data defined against 
a different schema, one would just change these variables or keys.
Any suggestions on how best to tackle this?

Bruce