BLACK IS BEST by Jack Olsen. 255 pages. Putnam. $4.95.
In the past seven years, Cassius Marcellus Clay Jr. has fought 28 of the best-publicized fights in boxing history. Some of his victims were stiffs, but most of them were decidedly more skilled than Clay's critics would admit. Nobody today denies that he is a superb boxer, but Clay himself beclouded that fact long ago in a great golden haze of self-generated mythology about his life outside the ropes—his ridiculous, irreverent verses, his portentous prophecies, his jazzy clothes, his religion, his wife, the draft...