If so, you need:

<xsl:variable name="bibrecord"
  select="$bibkey/doc(concat('bib-data/', ., '.mods'))" />

because in this case the <mods:mods> element is parentless. This
distinction is very subtle, and usually unimportant, but in general my
advice would be to avoid the "//" at the beginning of the pattern
unless you're really sure that you'll never want to match elements
that don't appear in a tree rooted with a document node.

Cheers,
Geert