Is there a code library/API available that customers can use to customize the search engine?

Customers do not have access to the search engine algorithm or code; however, customers can customize many indexing and serving features in the Admin Console. For example, customers can customize the crawling schedule, crawling patterns, and keyword matching. Customers can also modify the search results using the XSLT style sheets available in the Admin Console. You may want to review the Search Protocol document here: Search Protocol document here: http://code.google.com/enterprise/documentation/xml_reference.html.