Producer Scott Rudin has bought film rights to “L.A. Rex,” a crime novel by LAPD antigang investigator Will Beall.Deal comes a few weeks after Beall sold Riverhead Books a pair of novels set in South Los Angeles — “L.A. Rex” and “The Lion Hunter.” Both books portray the police and criminal cultures of the volatile district. Rudin recently announced he’s planning to depart Paramount for Disney when his producing deal concludes next year but it’s uncertain which projects he’ll take with him. Rudin’s next project is expected to be for Par: a romantic comedy “Failure to Launch,” starring Matthew McConaughey and Sarah Jessica Parker.