Boxall's 1001 Books You Must Read Before You Die discussion

Dreiser is on the list for Sister Carrie, but his other classic, An American Tragedy is well worth the time too. Cannot understand why it is not on THE LIST.

However, this book IS on the Guardian's 1000 Books list.

Sister Carrie broke moral boundaries by having an amoral woman not suffer the consequences of her neglect of the social values of the time. For example, she effectively becomes someone's live in lover without any negative consequences and the writing makes no comment on this behaviour. Ultimately, despite her involvement in things that would have made contemporary society blush, she becomes a success.

I think that's what justifies Sister Carrie's place on "THE LIST". I've not read An American Tragedy though so can't say what would justify it's place. It has to be something more than just the fact that it might be "worth the time." After all, there are plenty of 1001 novels that are definitely not worth the time (he says as he nears the end of Against the Day by Thomas (pointless) Pynchon.