Lady Susan is the earliest of Austen's novels, and in my opinion the weakest. (Really a novella, it's only 23,021 words.) It was written in 1794 when Austen was still in her teens. I found it hard to get into at first. Unlike her other novels, this is an epistolary novel told almost entirely in 41 letters, not third-person narration. The story feels thin compared to her other works as a result, although about halfway through we got more of a sense of scenes, with actual dialogue.
It's not that I don't find it worth reading. This novel is very different in tone than Austen's other novels--her titular heroine is a villain--a catty and malicious adulteress trying to force her daughter Frederica into a marriage of convenience. But if I weren't an Austen fan, I doubt I'd have persisted in reading it far enough for the fascination of Lady Susan's machinations to take hold, although take hold they did. The ending nevertheless feels abrupt to me.
I understand Phyllis Ann Karr did a third person narrative adaptation of the story. Particularly since she's an author I've liked, I'd love to read that. Sadly it's long out of print.