Howard Mann -- whose business ventures include online nude gambling -- got Katherine Jackson to agree to publish the book, "Never Can Say Goodbye," scheduled for release this week.
Mann also told TMZ he and Katherine will make record deals for 273 unreleased Michael Jackson songs -- part of an MJ treasure trove he acquired in a fire sale after Joe Jackson failed to pay a storage bill.
But estate lawyer Howard Weitzman is ticked, claiming Mann "may be using his relationship with Mrs. Jackson to infringe upon Michael Jackson's copyrights ... to the detriment of, among others, Michael's 3 kids."
Weitzman says Mann has no right "to exploit any estate assets" and he will take "whatever action is necessary to prevent him from unlawfully profiting." TMZ translation -- the estate will sue his ass if Mann goes forward.
We're told Katherine Jackson is happy with the job the executors have done -- namely filling the coffers with hundreds of millions of dollars -- but sources say family members are in her ear, getting her to sign off on ventures that end up hurting the estate, MJ's kids, and Katherine herself.