Can someone point me to more pieces of information on the ancestral spirits than given here?
They are worshipped mainly by stone giants and taiga giants. Is there perhaps something more in the giants revisited book?
I don't want to buy it without knowing it contains something interesting for me.
I have chosen the ancestral spirits as patron deity for my half-orc scarred witch doctor with the ancestors witch patron.
Does anyone know what holy symbol they have?
First, I recommend never using Golariopedia. It is essentially abandoned and occasionally vandalized; the only reason it exists is that Wikia holds content hostage when a project leaves its network. The actual fan-created wiki resource for the setting is PathfinderWiki.
The project's Anscestral Spitits article has not been updated recently, but we generally make a point to note the location of additional information on talk pages until someone gets around to adding it to the article. Checking reveals there is some information on them in Giants Revisited; I have not read that book yet, but I doubt there is anything of any real depth on the topic. The books of this type generally focus on the creatures themselves, rather then their pantheons.
I recommend picking up Giants Revisited; the Revisited titles are generally well done and enjoyable, but I would not expect to find an in-depth analysis of the Ancestral Spirits inside. I could be completely wrong though; if I am I'm sure someone will correct me shortly. I'll also make a point to check the book the first chance I get.