Is Brad Pitt and Angelina Jolie's union on the rocks? Are they truly compatible? Author Ian Halperin talks about America's most fascinating couple in his new book, Brangelina: The Untold Story of Brad Pitt and Angelina Jolie to ET...

In the book, available starting today, investigative journalist Halperin promises to expose "what's true, what's not, and what really has gone on behind closed doors with [Brad and Angelina]."

Halperin tells ET, "Angelina and Brad are deeply concerned about the book, about what will come out, because they know I've been in touch with people in their camp."

He adds, "However, I think both of them will breathe a bit of a sigh of relief because the book, in some sense, is positive about them. I really admire both of their careers. They've lasted a long time in the business, and they're extremely serious artists and they're good humanitarians."

So, what does Hollywood's A-list couple have going against them?

"The problem is the issue of compatibility," explains Halperin. "I urge both parties to read the book because it will shed insight on both of them to each other."

Watch ET for more on Brad and Angelina!