Adler's Philosophical Dictionary: 125 Key Terms for the Philosopher's Lexicon by Mortimer J. Adler
Late in his career, Mortimer Adler produced this slim volume as a companion piece and summary of his previous works in philosophy. He defines 125 key philosophical terms, explaining how they are used correctly and the common errors people make about the terms. He also explains how those terms as ideas fit into his larger body of work, and indeed into the larger understanding of reality and human experience. Even though these are philosophical terms, the field of philosophy often touches upon or overlaps other fields, such as science, politics, and theology. His explanations of how they fit in with the other disciplines is insightful and helps the reader to understand Adler's larger world view (including his Christian faith). Occasionally the reader has to slow down or repeat a reading to get the nuances of Adler's definitions but the effort is well worth the reward. The book is barely 200 pages, so even a slow reading does not take a huge amount of time.
Sample Text, from the entry on Continence and Incontinence (which is used very differently in philosophical circles than in common parlance):
We cannot understand what it means to say that man is a rational animal without, at the same time, recognizing that a rational animal is a freak of nature. The nature of a rational animal is a mixture of incompatible elements. [p. 74]