Haven't tried Assimil's course, but I'd recommend Lily Kahn's excellent Routledge guide, Colloquial Yiddish, which aside from being pedagogically excellent is very funny and had me laughing a lot.

https://www.amazon.com/Colloquial-Yiddish-Lily-Kahn/dp/04155...

There are also many courses on offer around the world. One of the great things about them is they tend to also pass down Yiddish culture too, not just the language. Happy to help you find one and get you on track to read IB Singer!