SharePoint Search Results: Filter by Document Library

Question

I have a requirement to perform a search against Search Server Express that will return a list of search results filtered by Document Libraries. That is, I need to be able to identify Document Libraries themselves from within search results.

Currently, we are using FullTextSqlQuery to return a DataTable. The query currently looks like this:

SELECT Rank, Title, Description, Path, LastModifiedTime, IsDocument, SiteTitle, MetadataAuthor, FileExtension FROM Scope() WHERE \"scope\"='All Sites' AND FREETEXT(*,'{0}')

Obviously we're formatting the above to contain the query text.

My question is: How can I filter the results based on Document Libraries? Is there a managed property I can use that will help me correctly identify if a search result is a Document Library?