Only someone with the "Gift" is able to speak to the Spirits. Others cannot even see or sense them. Bayaz believes that Logen Ninefingers may now be the only person left in the Circle of the World with the Gift. Logen can call them by blowing strong liquor out in a mist over a fire. They generally arrive quite slowly.
The Spirits are timeless and have existed ever since the Old Time of Euz. They seem to be associated with a particular location, since Logen says that all the Spirits in Adua are sleeping now. Spirits also seem to be associated with elements; the Spirits in The North are associated with wood and trees ("voices like the thousand sounds of the forest"), while the Spirit in Shabulyan seems associated with stone ("a moving pile of grey rock as tall as two men"). They impassively observe the goings on in the world around them, although they have little interest in humans and are always weak on the details. As Logen says, seeking useful guidance from them is usually pretty useless.
As with magic, which is leeching from the world, the Spirits are fading and going to "sleep"; there are now few of them remaining active. In the whole series, Logen encountered four different Spirits, two of which had definitely faded while the other two said they now planned to.
Centuries ago, after the destruction of Aulcus and the fall of Glustrod, the three remaining sons of Euz met; Juvens, Bedesh, and Kanedias. They discussed what should be done with The Seed, and eventually agreed to journey to Shabulyan, and gave up The Seed to the Spirit of the island. It was to keep it safe, and only give it to someone who carried Juvens’ staff.
The Blade Itself Edit
Logen becomes separated from his crew, and finds himself somewhat directionless. Alone at a camp fire, he uses his Gift to contact the Spirits, more for the company than advice. Three Spirits appear to him and tell him three things: that he himself survived a fall from a cliff, that Bethod has made himself King, and that the First of the Magi, Bayaz, is seeking Logen.
Before They Are Hanged Edit
Bayaz and his companions journey to the island of Shabulyan. Logen calls the Spirit of the island, who gives Ferro the stone which it had carried in it's stomach for centuries. It's duty done, the Spirit fades away from the world. However, the stone turns out to be just a stone, a trick of Kanedias to keep The Seed for himself.
Last Argument of Kings Edit
Logen speaks to the Spirits again before his duel with Fenris the Feared. Only one of the three Spirit arrives, the others having faded. It tells him what it knows about the Feared. Before the Spirit disappears, it tells him that it expects to fade now, and Logen won’t see it again.