by RENATA ADLER
178 pages. Random House. $7.95.
Many people believe that a book called a novel will offer a group of characters moving along a plot line with something approximating forward momentum. Many are also equally certain that they have heard quite enough, thank you, about the miseries of Manhattan neurotics. Normally, such convictions are not only sound but healthy; when acted upon, they protect the wary reader from a good deal of gibberish and whining. Still, any critical principle worth holding is also worth ignoring if a good occasion arises. Speedboata...