I first came across The Madwoman in the Attic via Madeleine's thesis project in The Marriage Plot, but I'm surprised that I made it through four years of humanities studies without having read it, as I, much like the fictional Madeleine, spent a lot of time with books that were described as "discourses." The Madwoman in the Attic is the O.G. of feminist readings of fiction, and its influence cannot be overstated. I've been thinking a lot about it recently as I was reading Claire Messud's The Woman Upstairs, whose very title and premise are something of an homage. The book is largely concerned with female authors like Jane Austen, the Brontes, and Virginia Woolf, and it's a really excellent follow-up to anyone who has already read and enjoyed some of the major women writers of the eighteenth and nineteenth centuries.
It's easy to find a first edition at any decent used book store (especially those in college towns), but do yourself a favor a pickup a newer copy that include their revised introduction, which should give you a better taste of the cultural impact the book had on feminism and literature.