FLETCH WON by Gregory Mcdonald (Warner Books: $14.95). The irrepressible Fletch (nee I. M. Fletcher)--he of the quick wit and off-the-cuff one-liners--is back in action to the undeniable delight of the thousands of Fletch cult members who have sprung up around this most unlikely of newspaper reporters. The joy of any Fletch novel (and this is Mcdonald's eighth, not his ninth as was erroneously reported on these pages earlier), isn't really the plot (in this case, the question of who did in an oily criminal lawyer in the parking lot of the News-Tribune), but in the wonderfully bizarre situations in which the personable Fletch finds himself on his way to the solution of the crime. How on earth can it be made plausible that Fletch will, of course, be discovered by the police a few hours after the murder, swimming nude in the dead man's private pool? How is it made equally plausible that, throughout a major portion of "Fletch Won," our hero is darting about town in the skimpy shorts and body shirt that are his uniform as a towel-boy in a high-class brothel? Never mind that no rookie reporter covering a humdrum society story could get into so much trouble in such short order--with his editor, the paper's ace crime reporter, the police and his constantly frustrated fiancee--the important thing is that it works in this zany, good-natured romp through the funny side of crime.