Just in time for the Fall elections comes a pointed and polemical primer from the columns of Thomas Sowell. From the introduction: ''That Barack Obama in office has often done the exact opposite of what Barack Obama said as a candidate, on issue after issue, should not cause half the surprise and disappointment that it has produced in many people who pinned high hopes on him. ''The really painful surprise is that so many people based their hopes on his words, rather than on his deeds. ''However, our concern is not with one man but with a country. When we look back at the decades-long erosions and distortions of our educational system, our legal system and our political system, we must acknowledge the chilling fact that the kinds of dangers we face now were always inherent in these degenerating trends. These essays deal with these trends individually, but it may help to keep in mind that they were all going on at the same time.''