Trump won't get a book deal, because the only book he ever read was "Art of the Deal", and "Art of the Deal" was written by someone else at Trump's behest. In short, he reads and writes at the 11th grade level -- obvious from his sneering inhalations that punctuate his speech when he's following the teleprompter.
Trump never had to pass the bar exam. Trump never had to subscribe to the ethics requirements of the legal profession, even as we snicker at the joke that sharks don't eat lawyers out of professional courtesy.
The idea that someone who lied once cannot be trusted a second time to tell the truth is by itself a litigator's argument and not a scientific appraisal of human behavior. If, for instance, you have a mafia thug who lies in court, and you tape-record him subsequently in his own safe surroundings as he talks to other lying thugs, you can be sure that the thugs aren't lying to each other. And if he lied the first time, one should ask "why did he lie the first time?" because the answer to that question must be included as a factor to test the inference about him lying the second time.