dune - How true to the original vision of Frank Herbert were the sequels published by his son?

Brian Herbert and Kevin J. Anderson based their sequels to original Dune series on research material composed by Frank Hebert before his death.

Are there any sources that describe how much material was found and what it contained?

Anything that can help to evaluate how true both authors kept to the original vision.

Answer

I started with the Brian Herbert wikipedia article's references section. There are a lot of sources here but here are the ones I think are the most significant.

First there's this archived blog post by Kevin J. Anderson where you could see those disk :

To the left, I'm posting copies of the actual IBM diskettes containing the files, with the labels "Dune 7 Outline" and "Dune 7 Notes" in Frank Herbert's own handwriting.

And lower on the blog post you could read :

When Frank wrote HERETICS and CHAPTERHOUSE he obviously intended to complete the story. He had planned an entire epic that tied his chronicles back through his whole projected history.

Frank Herbert wrote a detailed outline for "Dune 7" and he left extensive "Dune 7 notes," as well as stored boxes of his descriptions, epigraphs, chapters, character backgrounds, historical notes -- over a thousand pages worth.

Second there's this interview with Brian Herbert

Q: Did Frank Herbert leave you any notes to guide you in writing these prequels?

A: When dad died he was using a yellow highlighter on Heretics of Dune and Chapterhouse Dune. That's all I knew. Then I got a call from an estate attorney who asked me what I wanted to do with two safety deposit boxes of my dad's. I didn't know they existed. So we went down there, and in them were the notes to Dune 7 -- it was a 30-page outline. So I went up in my attic and found another 1,000 pages of working notes. As far as filling in these stories, Dad would say for example, "Between Dune and Dune Messiah it's been eleven years and billions of people were killed in the jihad." That's very enticing. So we're staying right on the Frank Herbert thread of his story line.

The others references are almost the same, except that they all mention getting the notes box in the garage instead of the attic.

Finally, in this interview, there is a passage concerning the possibility of releasing the outline or the notes.

[Q:] Is there any chance to see Frank Herbert's outline for "Dune-7" (we mean the notes) or some other articles devoted to "Dune"? Are you going to publish them anyway? If so, may we have your comments on it?

[A: ] We’ll probably include some notes or correspondence in THE ROAD TO DUNE -- but if we publish the notes and outline, then people will know the whole story! We want readers to get all the details from the published novels.

But the content of the now released The Road to Dune don't mention anything about that, so we could guess they wont publish it since they wrote the books and I don't see any argument to publish it (from their point of view).

So they started from a 30 pages outline and wrote 2 book of 700 pages each. There's also the notes, but we could imagine must of it was background material and they also wrote all the prequels books. So, we could imagine they stayed inside the parameters defined in the outline and notes, but there was some larges blank spaces to fill.