How to count missing textual references using XSL

If I have an XML document as follows:

<info>
   <books>
       <book name="Book1" />
       <book name="Book2" />
       <book name="Book3" />
       <book name="Book4" />
       <book name="Book5" />
       <book name="Book6" />
   </books>

   <quotes>
      <quote from="Book1" />
      <quote from="Book13" />
      <quote from="Book4 />
      <quote from="Book2" />
   </quotes>
</info>

How can I get a count of the //quotes/quote items where there is no corresponding //books/book (using the @from as a pivot against the @name)?

I'm sure it's something to do with xsl:key but can't get anything to work.  The source-data is hand-coded and a schema will not suffice... it's not illegal (in my real-world case) for the data to be missing, I simply need to know how many are missing....

Thanks.
Ray