She's a British singer/songwriter who, in my opinion, has been steadily improving with each new record. Brilliant songs, great voice, and independent to boot - she's never had much commercial success, but is a big hit with the critics and has a loyal fan base.
Tori connection - Thea is a good friend of Neil Gaiman and appeared on the Neil Gaiman tribute album, contributing a new song, "Even Gods Do."
Thea has just released a new live album, Recorded Delivery, which I guess is a good introduction to her work. Rules for Jokers, Avalanche, and Harpo's Ghost are also all good starter albums.