According to Wookieepedia:
The Journal of the Whills was a legendary record of events in the galaxy. Ostensibly, the Journal was maintained by the Ancient Order of the Whills, a mysterious group of beings.
It was an ongoing recording, as the actions of the Skywalker clan were recorded some 100 years after the battle of Endor, as told in The Making of Star Wars: The Revenge of the Sith.
Similarly to other major fiction works (like The Lord of the Ring's "origins" in The Red Book of Westmarch), it was designed originally as an outline.
In his early drafts, George Lucas ostensibly planned to use the Journal of the Whills as a plot device to connect the Star Wars galaxy to our own. In Star Wars: The Annotated Screenplays, Lucas explains his original intent:
Originally, I was trying to have the story be told by somebody else; there was somebody watching this whole story and recording it, somebody probably wiser than the mortal players in the actual events. I eventually dropped this idea, and the concept behind the Whills turned into the Force. But the Whills became part of this massive amount of notes, quotes, background information that I used for the scripts; the stories were actually taken from the 'Journal of the Whills'.
The Whills are mentioned sparsely inside canon. One such place they are mentioned is in the Revenge of the Sith Illustrated Screenplay. There are two mentions inside the book. The first is in a scene not actually shown in the film:
On the isolated asteroid of Polis Massa, YODA meditates.
YODA: Failed to stop the Sith Lord, I have. Still much to learn, there is ...
QUI -GON: (V.O.) Patience. You will have time. I did not. When I became one with the Force I made a great discovery. With my training, you will be able to merge with the Force at will. Your physical self will fade away, but you will still retain your consciousness. You will become more powerful than any Sith.
YODA: Eternal consciousness.
QUI-GON: (V.O.) The ability to defy oblivion can be achieved, but only for oneself. It was accomplished by a Shaman of the Whills. It is a state acquired through compassion, not greed.
YODA: . . . to become one with the Force, and influence still have . . . A power greater than all, it is.
QUI-GON: (V.O.) You will learn to let go of everything. No attachment, no thought of self. No physical self.
YODA: A great Jedi Master, you have become, Qui-Gon Jinn. Your apprentice I gratefully become.
This forms the backstory I had never before heard of of the scene where Obi-Wan and Yoda talk for the last time before Obi-Wan leaves for Tatooine. The scene in the film does not mention the Whills at all, but from the Illustrated Screenplay again: (
Text struck through is that which does not actually appear in the film)
YODA: (continuing) Master Kenobi, wait a moment. In your solitude on Tatooine, training I have for you.
YODA: An old friend has learned the path to immortality.
YODA: One who has returned from the netherworld of the Force
to train me . . . your old Master, Qui-Gon Jinn.
But, how could he accomplish this?
The secret of the Ancient Order of the Whills, he studied. How to commune with him. I will teach you.
OBI-WAN: I will be able to talk with him?
YODA: How to join the Force, he will train you. Your consciousness you will retain, when one with the Force. Even your physical self, perhaps.
Though the Wookieepedia entry mentions the Whills being mentioned in the A New Hope novelization, I could not find it in there. The Whills are mentioned in at least one other EU book, the young adult novel Last of the Jedi #2: Dark Warning
Even as Obi-Wan ticked off their possibilities for escape, he wanted to kick himself down the spaceport for being here in the first place. He had been on Tatooine when he heard Ferus was in trouble...(snip)...But was saving Ferus enough of a reason to risk leaving Tatooine. Obi-Wan had been racked with indecision...until he heard his former Master, Qui-Gon Jinn, who had last spoken to him, thanks to Qui-Gon's training with the Whills.
Obi-Wan had to find the answers to those questions. And he wasn't going to find them in exile on Tatooine.
Or, he realized, in the Caves of Illum.
You must follow your feelings, Qui-Gon had said.
And suddenly, Obi-Wan had a feeling that Qui-Gon was with him. Free of the constraints of place, trained in the way of the Whills, Qui-Gon could be right beside him, and Obi-Wan wouldn't know except for the feeling that filled him.
If Luke is to rise, he must have something to join, Qui-Gon's voice said in his mind.
And then he knew, as surely as he knew his mission, why Qui-Gon had told him he wasn't ready for training with the Whills.
When you know why you are not ready, you will be ready, Qui-Gon had told him.
It is possible there are other references to the Whills in other EU works, but google books searches didn't turn up anything, or the full text was not avaliable (Tales of the Bounty Hunters and From the Adventures of Luke Skywalker).
From what I found, it is not clear exactly how much you could learn from the Whills, but the power of becoming a Force Ghost was clearly linked to the Whills, and there was clearly a path, an ideal, that they followed. Beyond this, I can't confirm any other information.