From "Refrain," 1959
Karlheinz Stockhausen

Search by composer's name:

• "Keyword search" for composer's name is simplest.
• "Author search" for compoer's name is most specific.
• Remember to limit your search to scores.

Names can have variant spellings.  The library catalog uses "Authority Records," which contain the standard form of a name and common variants to help cross reference various spellings.  This is not 100% accurate, so if you feel you may not be getting complete results, try searching on various spellings you may already know or consult with a librarian.

If you have no idea how a name is spelled, try searching Google and look for suggested spellings.
For authoritative standard spelling, consult the Oxford Music Online / Grove Dictionary of Music and Musicians.  This resource also cross references from variant spellings.