period genre exceptions

I'm looking for a tag for books that stood out at the time they were written - they have historical context, irrespective of whether they stand up to modern criticism.

I want to avoid 'classics' - because that usually denotes some form of worth, which I don't always agree with.

The specific book I'm looking at is The stainless steel rat which was apparently groundbreaking at the time. But has aged badly. There are a few similar examples.

Any thoughts?

How about:

momentous

I think you came up with it already :) "groundbreaking" describes it well.