The best book I've found for addressing basic regulations is the Federal Aviation Regulations Explained, by Kent Jackson. He stopped publishing the book on 119 and 135 regulation, I believe, and no longer puts out material on Part 121. His book on Part 61 and 91, however, is quite enlightening and covers not only explanations, but Chief Legal Counsel interpretations and actual legal cases.

Presently Kent produces three books, which are published through Jeppesen.