There are lots of secular textbooks and courses, but the dialect they teach didn’t make it past the mid-20th century, and contemporary Yiddish is as different from “YIVO” Yiddish, and modern English is from Shakespeare. Pre-war “literary” Yiddish was much more influence by German, and also a lot less “clean” than the dialect spoken by Bnei Torah.
Assuming you already know Hebrew and English, but not German, I’ld suggest hanging outsomewhere where children speak Yiddish on the street. You can build some vocabulary from secular books, or from translating a newspaper or children’s books.