The library also has a copying service if you know what you need. They provide several other services as well. Several major libraries have their catalogs available on line. Library of Congress Including an on-line catalog search facility. Here is the results of a search for Pennsylvania genealogy books .
Some genealogical publishers have their catalogs on line.
The list below is in no particular order. Links last verified as working 15 Oct 1999. Sometimes you can find out of print books at Antiquarian Booksellers. Here a list of places where you can find searchable on-line catalogues for antiquarian books. These links were verified as working 13 Jan 1999.