Following are several books that I've found useful, although several are out of print.  Many of these can be found at amazon.com or alibris.com.  Occasionally, the books can also be found on ebay.


A Pennsylvania German Reader and Grammar, Earl C. Haag, Keystone Books, The Pennsylvania State University Press, 1982.
A Simple Grammar of Pennsylvania Dutch, J William Frey, Brookshire Publications, Inc., 1985.
A Pennsylvania German Grammar, Albert F Buffington and Preston A Barba, Schlechter's, 1954.

The following is more of an introduction to the language than a grammar book.  Still, it is an excellent introduction and includes lots of pictures (so you may learn better the Pennsylvania Dutch word for a cow when it's next to a drawing of a cow, if you're visually inclined).
Introduction to Pennsylvania German, Elizabeth Wengerd, Millersville University, 1993.

Pennsylvania German Dictionary, Eugene S. Stine, The Pennsylvania German Society, 1996.
Pennsylvania German Dictionary, English to Pennsylvania Dutch, C. Richard Beam, Millersville University, 1994.
Pennsylvania German Words in Context, C. Richard Beam, Millersville University, 1997.
An recent email announcing a new book from Millersville.

A collection of dialect stories, poems, etc. but good material to practice reading (no English translation!)
A Pennsylvania German Anthology, Earl C. Haag, Susquehanna University Press, 1988.

The following two have subject matter useful for teaching children (and adults).  I recommend buying the CD soundtrack (available with purchase of the Mammi Gans book).  click here for details
Mammi Gans, Mother Goose's Nursey Rhymes, Pennsylvania Dutch and English (and CD soundtrack), Inkwell Editions, 2003.

Hans un Yarick, Bilingual Edition, Walter Sauer, Inkwell Editions, 2002

The following is considered a classic of Pennsylvania German wit.
Pennsylvania German Boonastiel (with English translations), T.H. Harter, Schneider Printing Company, 1928.

En Glee Bissell Des un Sell, occasional column in the East Penn Press.

Daheem uff die Bauerei, occasional column in the Journal of the Berks County Genealogical Society.

Dialect Columns, The Pennsylvania German Review, periodical of the Pennsylvania German Cultural Heritage Center, Kutztown.

Pennsylvania German Secular Folksongs, Albert F. Buffington, The Pennsylvania German Society, 1974.
Songs along the Mahantongo, Boyer, Buffington and Yoder, Intelligencer Printing Company, 1951.


The following has some interesting articles on linguistic research in the dialect and includes geographical analysis of variants.
A Word Atlas of Pennsylvania German, Lester W.J. Seifer, Max Kade Institute, 2001

The following is an interesting early analysis of the dialect including it's origins in Germany.
Pennsylvania Dutch: A Dialect of South German with an Infusion of English, S.S. Haldeman, 1872, reprint by Evolution Publishing, 2003.

Also, I'd recommend any article by Marion Lois Huffines, a well-known Pennsylvania German language researcher from my alma matter, Bucknell University.  She has published numorous articles on the dialect itself and the decline in its usage.