Okay, so she's not an unpopular author but I really love all Trudi Canavan's books and I can't believe she's not more popular. She creates such an awesome mix of fantasy and romance in her stories I can't believe more people don't read her works!
I've only read Mark's book, The Recluse Storyteller, but if that's anything to go by his other books are absolutely fantastic. It may not sound like much, but his story really grabbed me and was just beautifully written! My review: The Recluse Storyteller
I haven't read all of Isobelle's books but I have read a fair few of them with my favourite being Alyzon Whitestarr (which is a very underrated book in itself!) and I think her books should be more popular than they are!