I haven't read the book but I heard that it is purely based on gossip, and no new gossip afaik. They just repeat everything that has been said about the last ten years or so.
Both Promi and Privat (they have even worse headlines) are magazine that shouldn't be taken too seriously, they are yellow press that make headlines up to sell. And they are not just rude towards the Grand Ducal Family, they are rude against most of the famous people living in Luxembourg, quick in pointing out mistakes made by others and sometimes think that they are the most important publication. For example a few weeks ago when the Grand Duke had his angioplasty Promi insisted on the fact that if they hadn't published the protocol of the emergency service, the Court would have never released the info about the angioplasty. I'm sure the Court doesn't feel compelled to release information just because a gossip magazine writes something on their website. I haven't read too much of Promi, but what I know by occasionally reading other yellow press is that there usually isn't much behind a sensational sounding headline
The more respected magazines in Luxembourg handle the Grand Ducal Family differently. They just write about some engagements they did and sometimes about possible future wives for Guillaume.