Although his books were being done by another publisher, the legal thrillers of D.W. Buffa were so alluring to Putnam senior editor David Highfill that he never ceased to remind Buffa's agent, Wendy Sherman, of his continuing interest. After several years, during which Buffa's books were successfully published by Warner, his patience was rewarded last week when he finally persuaded Sherman to accept a mid-six-figure offer for North American rights in Buffa's next. Editor and agent were both so gratified that they called "Deals" separately to stress their enthusiasm about the new relationship. "I'm delighted," said Highfill. "This was truly meant to be," said Sherman. Buffa's novels center around West Coast lawyer Joseph Antonelli; Warner still has The Legacy to come next summer, as the last book under the old contract.