We start playing the Wrath of the Rightous campaign and are looking for detailed information about Mendev and Kenabres but cannot find anything exept from the campaign guides and the Campaign Setting book. Are there any other sources where to find more information?
There aren't any specific books published yet for Mendev that I am aware. You might want to check out the Worldwound sourcebook that would have come out around the time of the Wrath of the Righteous adventure path. I'm not sure if you have the old Campaign Setting vs the Inner Sea World Guide but they might have slightly different information as well.
As far as I could find Kenabres is detailed specifically in the adventure path itself.