The characters that filled the pages of the three earlier ''Tales of the City'' books with love and laughter are at it again, as an ordinary house-husband and his ambitious wife discover there's more to making a baby than meets the eye. Unexpected help arrives in the form of a British monarch, a grieving gay neighbour, and an international ring of mail-order brides. Armistead Maupin has written a comedy of manners for our times.