Errol Morris received an unexpected endorsement Monday from Sarah Palin, who posted a positive critique of Morris’s new book about the Jeffrey MacDonald case, “A Wilderness of Error,” on her Facebook page. (MacDonald is the Green Beret doctor convicted of the 1970 murders of his wife and daughters, and the subject of Joe McGinniss’s 1983 bestselling book, “Fatal Vision.”) In his book, Morris, who lives in Cambridge, contends that prosecutors botched the case against MacDonald, and he makes a persuasive case for MacDonald’s innocence. So why is the 2008 Republican vice presidential candidate weighing in on a decades-old murder case? Simple: Morris’s book raises serious questions about McGinniss’s book, and Palin is no fan of McGinniss. After all, he’s the author of “The Rogue: Searching for the Real Sarah Palin.” “What McGinniss did in my town and to my family was sick and vicious,” Palin writes. “I sympathize with MacDonald and his defense team because I saw firsthand the twisted way McGinniss operates.” Reached Monday in North Carolina, where he’s attending a hearing related to MacDonald’s bid for a new trial, Morris said he hadn’t yet read Palin’s review. “But it’s gratifying because it is bringing the book and, more importantly, the case to a much wider public,” he said. (It’s also ironic because Palin and Morris are not politically like-minded.) And what did her review do for sales? “I know, anecdotally, that my book is climbing rapidly in the ranks as a result of what she wrote,” said Morris. Considering that Palin has 3.5 million fans on Facebook, he shouldn’t be surprised.