re: Dune

Start with the original Frank Herbert novels. Unfortunately, Frank died before finishing the series. But it is all worth reading, and is my favorite Sci Fi series.

His son, Brian, wrote Dune prequels and attempted to finish the original series. You'll find mixed reviews on Brians books, but most people who read Frank's books first hate Brian's mess.

The books definitely weren't written for teens. It deals a lot with sociology, psychology, philosophy, and the nature of finite resources. The underlying theme of the books is mainly "Beware your heroes."

Don't go into it expecting a lot of action. Most of the action takes place "off screen" and is often just mentioned in passing.

The original series is six books:

Dune
Dune Messiah
Children of Dune
God Emperor of Dune
Heretics of Dune
Chapterhouse Dune

They take place over millennia.

The prequels written by Brian Hebert are the "House" books (House Atreides, House Harkonnen, House Corrino) which take place before Dune and cover Leto Atreides when he was a young Duke.

The pre-prequels, or the Machine Crusade books are set thousands of years before Dune. They deal with the war against the Thinking Machines to free humanity from slavery. They tie directly into the two books Brian wrote that supposedly finished the original series.

Brian has also written other books set in between the original Dune books.

The House books weren't bad, and kind of flesh out the universe. But there are some serious inconsistencies between them and the original series.

The Machine Crusade books were not that good, and had even more serious inconsistencies.

The two books he wrote to finish the original series were so bad that it made me irrationally angry.