Writer Siegal and trainer Margolis again collaborate, not only to restate their basic theory-andpractice so well set forth in Good Dog, Bad Dog (1973), but to succor those owners not usually addressed in other dog manuals -- the mutt fancy. The main difference, they say, in educating the mixed breed as opposed to the purebred, is that genetic temperament -- fairly constant in each particular species -- must be analyzed early on to facilitate training. The authors run through general personality profiles -- shy, nervous, ""sedate,"" aggressive, etc. -- and append suggestions for each category. Some critics are sure to mention that these temperaments occur within each AKC breed and that a breakdown by body types (the stubby and compact; the big-pawed and rangy, etc.) would be more to the point. However, at least the authors are giving that good unsung American mutt his due, and their training tips are still sensible, humane and appealing. ""Commandcorrection-praise"" is the regimen that should calm the owner and make lessons more than a ""royal pain in the haunch"" for the dog. A supra-jovial style is not out of place (novices with piddling pups need all the cheer they can get) and the photographs (with trainers in white tie and tails) are useful and fun. Spring a doomed pup from the pound today.