This textbook by noted Yiddish linguist Dr. Mordkhe Schaechter is suitable for students who have completed at least 3 semesters of Yiddish. 
Yiddish II, written mostly in Yiddish, is the first continuation-level Yiddish textbook ever published which specifically addresses the difficulties encountered by English-speaking adults. However, speakers of other languages with only a passive knowledge of English can utilize this book with equal profit. It contains a wealth of grammatical information and is a must for any serious student of Yiddish.