The God Emperor story is by far the most accomplished part of the Dune universe. It is the one book from the extended series (outside the first book) that is really worth its salt.

The God Emperor sacrifices himself for the "Golden Path" that is never explained. After more than 3000 years he chooses his heir and prepares her for the troubled times to come, however, the story ends before we find out how things work in his absence.

There are many wise fragments that are worth quoting from this book, and I think this is where Herbert shows his strengths as well as his limits. Unfortunately, we never get a clear description of the Golden Path, but it is suggested that the Golden Path consists in hatred for stagnation, which the God Emperor instills in his subjects.

Worth reading and re-reading as well.