The critical reviews have of Jimmy Carter's new book, ''Palestine: Peace Not Apartheid.'' by now been widely circulated. I want to deal here less with the book than with the curious character of its author. Jimmy Carter is a virtuous man, no doubt about it. But he is not full-time virtuous. His virtue would be more compelling if he confessed, now and then, to self-doubt. Much of his indictment of Israel is based on fact, but almost all of it either lacks or distorts context, rendering it less than true. Facts are, by definition, true, but they are not the truth. His revered Holy Land deserves better; so do his reputation and his virtue.