A book about General George S. Patton, written by his unabashedly admiring nephew, manages to transform the potential weaknesses of a relative writing about a controversial figure into real strengths. In his ""Portrait of a Soldier,"" Mr. Ayer has made to no attempt to fully assess the General's career; rather, it is the man, ""Uncle George,"" whom he has attempted to commit to these pages. He had done so to often memorable effect. He relies mainly upon his own recollections, but also turns to those of other members of the family and former comrades-in-arms. We observe Patton in privacy, and gain some intimate new insights--who, for instance, would have guessed that ""Old Blood and Guts"" wrote poetry and devoutly believed in reincarnation? But mostly it is not so much a matter of learning about him as seeing, very nearly hearing, this man who, more than any other American in the past century, was the complete soldier. The colors, most fortunately, have not faded yet.