What's the first noteworthy Gothic book?

[Des Esseintes]

Since Walpole's CASTLE OF OTRANTO was first printed in 1529, I would surprised if it weren't the first notable one. I do not find them hard to read. They are not written like Shakespeare's plays.

Castle of Otranto isn't actually as old as Walpole claimed. It was actually first published in 1764 and written by Walpole himself. He claimed it was much older but that was part of the fiction. I read it a couple years ago and found it very readable. Some of the comedy of it hasn't aged well so that some readers take seriously those parts that Walpole intended to be lighthearted, but overall it was much easier to read than I expected.