''Significant Others'', the fifth self-contained chronicle in the ''Tales of the City'' saga, is a cunningly observed class comedy that's sure to be relished by the cognoscenti and by new readers alike. A holiday in the redwoods goes uproariously awry when the opposing sexes camp out rather too close to each other for comfort. Among those entangled in the mayhem are DeDe Halcyon, reformed debutante, troubled house-husband Brian Hawkins, and the irrepressible Michael 'Mouse' Tolliver (arguably Maupin's most beloved creation).