I rarely read "tell-all" books because they rarely do tell all. Oftentimes, when they do, it's usually a sanitized version of stuff people have already heard fifth-or-sixth hand through the rumor mill (e.g. Paul Lynde, Rock Hudson, only because the politics of the culture change enough to where more detail can be given). Every tell-all book has to go through legal advisory teams to ensure the author doesn't open themselves up to lawsuits. And that's just for starters.
A true tell-all book would violate numerous confidentiality agreements, thus risking years and years of litigation and bankruptcy, not to mention immediately burning the bridges of virtually all professional contacts. An A-list star could find themselves unable to get a job as a waiter in any field-friendly restaurant from coast to coast. The people in the entertainment industries have some dark, dark, DARK secrets that can get a book taken off the shelves, corporations severed of their property, and people killed. Now, this isn't everyone, but every piece of information out there can help in numerous ways.
Notice that even in tell-all books, people go out of their way to not name specific names. John Lithgow did that with his memoirs, but he hinted at the year, the project, and the actor's role well enough that I could reverse-engineer it through the IMDB (the actor who sabotaged other actor's takes to get his coverage more merit, BTW, was Cliff Robertson, who was the original Uncle Ben in Spider-Man), but Robertson was dead by then. Still, when you have to protect the names of the dead to ensure the living don't treat you as a liability, it goes to show what people have to hide in the industry and why we'll never really know what's what.
