I'd recommend starting at the beginning of any of her series, although I think her WOmen of the Otherworld books are the best. WotW and Darkest Powers are both set in the same "universe" but not specifically linked other than that the same species of supernaturals appear in both sets, although none of the same acvtual characters. The Nadia Stafford series is completely different - firmly set in "this world" and following an ex-cop turned assassin (very exciting stuff!). If you don't like supernatural stuff, go for the NS books. If you do like supernatural stuff, read the WotOW books first. Then read the NS books. Then the DP ones. They're all good, but in different ways.
Women of the Other World
1. Bitten (2001)
2. Stolen (2002)
3. Dime Store Magic (2003)
4. Industrial Magic (2004)
5. Haunted (2005)
6. Broken (2006)
7. No Humans Involved (2007)
8. Personal Demon (2008)
9. Living with the Dead (2008)
10. Frost Bitten (2009)
Spin off from WotOW
Men of the Other World (2009) (collection of shorts)
(this features back stories to some of the male characters in the WotOW series which slot in between some of the other novels)
Darkest Powers
1. The Summoning (2008)
2. The Awakening (2009)
3. The Reckoning (2010)
Nadia Stafford
1. Exit Strategy (2007)
2. Made to Be Broken (2009)