Could you, please, look up The Darkest Powers series?
The series contains two trilogies so far. They are a part of the same world.
The Darkest Powers trilogy
1. The Summoning;
2. The Awakening;
3. The Reckoning
+ Darkest Powers Bonus Pack (0.5 Dangerous; 1.5 Divided; 2.5 Disenchanted)
+ Enthralled - “Facing Facts” / Darkest Powers, narrated by Chloe; Appearing in Enthralled (http://www.kelleyarmstrong.com/books/)
Darkness Rising trilogy
1. The Gathering
2. The Calling
3. The Rising
As you can see the author explains it herself:
"Q: Will The Reckoning be the last in the teen series?
A: It’s the last in the initial Chloe trilogy. Book 4 in the “series” will move to different teens in the same world, subjects of another Edison Group experiment." (http://www.kelleyarmstrong.com/faq)
"Q: Why were plot threads left dangling in The Reckoning if it’s the end of the trilogy?
A: Because it’s the end of the first trilogy, not the end of the series. " (http://www.darkestpowers.com/questions/)
"..The Reckoning is the last book in the trilogy, not the last in the series. It only wraps up the major plotline introduced in The Summoning.
..The next book is The Gathering. Same world. New characters. New Edison Group experiment. The Gathering. starts trilogy #2, Darkness Rising. Chloe and the others will be introduced late in that trilogy." (http://www.kelleyarmstrong.com/forum/index.php?topic=29132.0)
So The Summoning is book one in the main series, AND book one in the first trilogy. And so on.
The Gathering is Book one of the Darkness Rising trilogy, but it's also book 4 in the Darkest Powers main series.
There are two short stories that are set in the Darkest Powers world, but they are not a part of any of the trilogies:
“Kat” - Darkest Powers, narrated by Katiana; Appeared in The Eternal Kiss • July 2009 • ed. Trisha Telep (http://www.kelleyarmstrong.com/books/)
“Hunting Kat” - Darkest Powers, narrated by Katiana; Appeared in Kisses from Hell • August 2010
- Hunting Kat (http://www.shelfari.com/books/25484752/Hunting-Kat)
- Kisses from Hell
Wow, I got carried away :)