Hmmm - this is a good question! And I am sure I have a ton that I have felt this way over the years - but this is what comes to mind now :
1) Dianne Wynn-Jones - LOVE HER. For anyone that loves Harry Potter, and childrens fiction - they should pick up The Chronicles of Chrestomanci. I do not know why she is not as popular as Rowling - but without doubt she should be. I think the real problem is her books were written back in the 70s/80s before the marketing machine that made Harry famous was in place. Her books are a tiny bit more old fashioned then Harry Potter -but since it is set in a magical world, it works as timeless fiction. Author's such as Neil Gaiman have done homage to her.
2) Joe Hill - A great horror novelist - and the next Stephen King.
Oh and he actually happens to be Stephen King's son. I would not have
thought writing talent is something that could be handed down from
father to son - but this is just eerie. He is really good - regardless
of who his dad is.
3) Elizabeth Van Armin - I saw the film 'Enchanted April' and really wanted to read the novel - because I was going to Italy. So I finally tracked down 'Enchanted April' and loved it.
On top of this - I saw an old Bette Davis movie called : Mr. Skeffington - very dramatic - typical Bette Davis fare. This also turned out to be adapted from a Van Armin novel
I never could find any more of her books - but I stumbled into them at a used book store. I found the book 'Love' and 'Vera'. (These titles are not even found when searching Amazon or Library of Congress on Librarything!) I also have 'Elizabeth and her German Garden' Still searching for a copy of Mr. Skeffington.
Not sure how she fell into obscurity - but Enchanted April is well worth reading if you like novels from the 20's/30's, between the wars time period.
4) Susan Hill - I first was introduced to her when I saw the one man play 'The Woman in Black' in the West End in London. I loved the play so I tracked down the novel - and found it to be a creepy little ghost story. She also had another ghost story - 'The Mist in the Mirror'. But the book that really impressed me was 'King of the Castle'.....Evidently in England this title is something that kids read in school - ala 'Lord of the Flies'. But it is not known here in the US. (I had to get it from Amazon UK) Very disturbing and psychologically tense - and I would highly recommend that book to any book clubs - because I remember being shocked by the 'villain' and wanting to talk about the book after I completed it.
I am sure I will think of some more authors later in the day. If so I will update this post!