I'm not saying you haven't read them, but odds are you haven't. Most certainly aren't best-sellers, and some of them are overlooked by the academy as well (at least as far as I know). Most are fiction writers (since that's my area and all); some I recommend on the virtue of one book, others on a body of work. Here, then, in alphabetical order:
1. Isabel Allende
Two books come to mind: House of the Spirits and Eva Luna. Allende's commingling of Latin American history and magical realism, coupled with a sure sense of story pacing, has earned her a justifiably strong reputation, but I find few people read her since the newness of El Boom has worn off in the Northern Hemisphere. By all means, read her.
2. Frances Burney
Jane Austen devotees may know of Fanny Burney, as there is a long-standing dispute between the readers of both novelists as to who's better. Can't we all just get along? but that being said, if forced to choose, I would set aside Austen (whom I like with reservations) for the more raucous, expansive (and I think funnier) Burney. Evelina is where to start.
3. John Crowley
Best contemporary American writer nobody's heard of. I recommend the short stories, Aegypt, and the masterful Little, Big, which is one of the best novels of the last 30 years. Started as a fantasist, which is why he is only now getting street cred in a highly biased American academia.
4. David Halberstam
Excellent journalist and historian of American culture. His work on Robert Kennedy is worth a read, but it was October 1964 that drew me. One of the most engaging books I have ever read, exploring the two World Series teams and the simultaneous events of the Civil Rights Movement.