Winner of the Stephen Leacock Medal for Humour, Leaven of Malice is the second novel in Robertson Davies's much-loved Salterton Trilogy, now part of the new Modern Classics series.      The following announcement appeared in the Salterton Evening Bellman : "Professor and Mrs. Walter Vambrace are pleased to announce the engagement of their daughter, Pearl Veronica, to Solomon Bridgetower, Esq., son of..."           And not a word was true. Although the malice that prompted this false engagement notice was aimed at three people only--Solly Bridgetower, Pearl Vambrace, and Gloster Ridley, the anxiety-ridden local newspaper editor--before the leaven of malice had ceased to work, it had changed permanently, for good or ill, the lives of many of Salterton.           This is the second novel in the Salterton Trilogy, which also includes Tempest-Tost and A Mixture of Frailties , and is another brilliant display of Robertson Davies's legendary wit and erudition.