The authoritative edition in the US is apparently the Vintage International Vintage Books edition translated by Ralph Manheim. I do not know if this is in fact the edition used for ereaders, but Amazon does have a kindle edition.
I don't know about other European editions--but what I've been kicking myself over is that critical guide I wanted. I came across it in an obscure website notation and haven't been able to find it since.
I regret that I really could not bear down and focus on a decent discussion in the forum book club here. I would have had to drop everything to reread Grass, and his diction even in translation is so rich, dense, it demands the respect of one's critical abilities. There are teaching guides available, but I want the above mentioned if I can find it.
It is easy to imagine that teaching The Tin Drum could take at least two semesters, if not more.