Wacker has a library available to anyone who has a Wacker employee number. However, most U.S. employees stay on U.S. payroll and will be unable to receive a library card.
Dr. Naue located on Robert-Kochstr. 36-38 Burghausen (as well as locations in Altötting and Neuötting) has a few English books available as well as stationery, office supplies, and cards. You can order any book you would like; compare prices with amazon.de to get the best deal. It takes a week or so for the book to arrive from Naue. Check back with them as they may not call you.
Frauendorfner bookstore on Marienstr. in Altötting has a really nice selection of English paperbacks. They will also order books from the Bestsellers list for you at no risk - you can look at them after they've arrived and then decide whither or not you want to order them.
Ordering books via amazon.de is also an easy and relatively inexpensive way of getting English speaking books. By the time you add shipping and customs duties, it's generally less expensive to buy from amazon.de than it is from amazon.com. Another advantage with amazon.de is that you can pay directly from your Girokonto.