A detective story is a Who Done It. A pennant race is a Who'll Do It. One starts with a dead body. The other ends with one. In the thriller, the cause of death customarily is murder. In baseball, usually it's suicide.

Clues and characters are scattered through the first 200 pages of Trent's Last Case so their accumulated richness can create suspense for the final 50. We bide our time in baseball, too, through 130 games from April onward, just so, come Labor Day, we can be fully informed and perched on the edge of our seats for the last 30 games of denouement.

Now, we're ready for baseball's concluding chapters of 1985. It's a book with a lot of promise. When did baseball last have more intriguing pennant race, playoff and Series possibilities?

With the Los Angeles Dodgers and California Angels both leading their divisions, the first freeway World Series is not farfetched. Watching the Dodgers' Tommy Lasorda manage against Gene Mauch would be too much fun to bear. The best dumb manager against the worst smart manager.

Chavez Ravine and The Big A would just be too nice in a year when the Series has the latest scheduled closing date in history -- Oct. 27. How can the demons that ordain such things pass up the chance to finish the season in hideous, exposed, blustery Exhibition Stadium in Toronto between snowstorms?

New Yorkers have noticed that the Mets and Yankees both are in second place with solid chances to create the first subway Series since 1956. Would Dwight Gooden (20-3), the best child pitcher ever, against Ron Guidry (16-4), who has the best winning percentage in history, attract a tad of interest?

If all these are dream Series, then we have potential nightmares, too: an All-Canada or an All-Missouri Series. Given the alternatives, this might not be the year to send up prayers to see Blue Jays vs. Montreal Expos or St. Louis Cardinals vs. Kansas City Royals.

Thankfully, the Expos -- whose Olympic Stadium is almost as ugly as Toronto's park -- already are staggering. The first meeting between the Cardinals and Royals is more plausible.

This has been a uniquely good year for managerial contrasts.

In the National League West, Tommy Lasorda, who prefers cheerleading to chiding, seems to wear far better with his players than acerbic Dick Williams, whose welcome with his defending National League champion San Diego Padres may be wearing thin.

In baseball more than any major sport, the easy-riding, don't-gimme-a-headache-with-strategy type of manager has a chance to endure and prosper. In a free-agent era when dynasties are defunct and change is the rule, it's vital that a manager know how to weather periods of prolonged defeat. After all, since 1978 only one division champ (the 1980-81 Yankees) has repeated.

If any race ever held promise of offering a fascinating managing test, it's the September show between the Cardinals' Whitey Herzog and Davey Johnson of the Mets. Herzog may know the game better at more levels than anyone alive, and he can milk mediocre bullpens with the best. Johnson is the prototype of the New Manager -- half-jock, half-brain. A four-time All-Star player, Johnson also has a college degree in mathematics and understands novel stat theories and computer use better than his peers.

Johnson already has shown amazing restraint for a man who's never managed in a race. When others might use an ace such as Gooden on less rest, Johnson is insisting that he have more, giving the 20-year-old five days off this week.

Alarm bells went off in Johnson's brain when he saw Gooden have a poor game followed by a 16-strikeout, 143-pitch masterpiece followed by another poor performance. That spelled: "tired young pitcher." In three previous pro years, Gooden has worked 79, 191 and 218 innings. This year, counting postseason, Gooden might be the first pitcher of his age (in lively ball times) to be asked to pitch 300 innings. Any arm can be ruined.

What if Billy Martin leads the Yankees into the playoffs? Will he meet the Angels and old buddy Reggie Jackson? Would he face his former coach, Dick Howser of Kansas City, probably baseball's most underrated manager? What about Martin vs. Lasorda or Martin vs. Herzog rematches that would recall battles of the late '70s?

It properly could be said that last year the entire regular season did not contain a single game of any true dramatic significance. It will take bad luck to foil our fun this time.

Can the Toronto pitching staff, gasping for a fourth and fifth starter, use its fabulously deep bullpen to hold off the voracious Yankees and their 850-run offense? If run differential is the true measure of a team's quality, then nobody in baseball is nearly as talented as the Yankees and Blue Jays. Both have a chance to outscore opponents by 200 runs.

Can the Cardinals, with Jack Clark already injured, ignore their complete lack of bench strength and their novice bullpen? The Mets almost have a monopoly on the established stars when they meet the Cardinals, but St. Louis has a three-game lead.

Will the Dodgers' starting pitching depth immunize them to the plague of collapses and near folds that have struck NL West leaders in the '80s. Could the Padres' old-pros make one last run at the Dodgers' kids?

And what about the closest race at the moment -- in the AL West, where the Royals want to beat the repeat jinx? If one man deserves our sympathy in coming days, perhaps it is Mauch of the Angels. His record (20 finishes of fourth or worse in 23 seasons) is a much-beaten dead horse. So are his tendencies toward archaic one-run strategies. This is a bright, decent man who has had enough unhappy endings.

Twice a baseball season's final chapter has ended with Mauch as the corpse in question. Both times, with the 1964 Phillies and the 1982 Angels, death by self-inflicted wounds was the cause of death. If Mauch finally reached a World Series after his career seemed buried, that would be a surprise ending, indeed.