If beginnings were all that counted in storytelling, Stephen King would be pop fiction's most reliable entertainer. Every one of the four long horror tales in this hefty (763 pages) collection, Four Past Midnight, lifts off handily within the first few pages. There's a likable character in some everyday situation borrowing a library book, getting a birthday present. Then, quite suddenly, the all- American atmosphere sours; something diseased intrudes. The oldest formula around? You bet. But King recycles it with a winning combination of old-time enthusiasm and up-to-date details. Nobody grabs you faster. Nobody begins better.
Unfortunately, stories also have to have middles and ends, and in King's less-controlled yarns, the middles sag while the endings often collapse into confused hysterics. With one exception, these new novellas represent the prolific King at his least royal.
''The Langoliers,'' for example, starts out strong: 10 of the passengers on a jet from L.A. to Boston fall asleep soon after takeoff and wake up three hours later to find that everyone else aboard has vanished. What's going on, you want to know? Well, you may be sorry you asked. Because, about halfway through this overgrown Twilight Zone episode, it's explained that somewhere around Denver, the plane flew into the past through a ''time-rip.'' Furthermore, when the survivors manage to land at Bangor, Maine, they realize that something is coming to get them. The ''langoliers,'' you see, are black and red spheres that gleefully devour everything in their path. Along the way one of the passengers says, ''It seems like one of those stupid disaster movies.'' Says another, a few pages later: ''I've flown into a bad made-for-TV movie.'' They're both right.
''The Library Policeman'' finds King in much more congenial company: ghosts, demons, the evil behind small-town facades like that of the Junction City (Iowa) Public Library. Sam Peebles, a cheerful local businessman, goes there to do a little research, and a plump, smiling, white-haired librarian named Ardelia Lortz helps him check out a couple of books. But when Sam can't find those books to return them, he receives a ghastly visit from Ms. Lortz's Library Policeman: a lisping, dead-faced giant in a trench coat. And then Sam learns that Ardelia Lortz who once terrorized the children of Junction City has in fact been dead since 1960!
So far, so good-or so serviceable, anyway. Despite stretches of truly awful writing, despite goony adolescent excesses, King makes us believe in she-devil Ardelia, who's determined to return to life on earth by taking over Sam Peebles' body. But King soon adds another, more pretentious layer to the story as Sam seeks a way to resist demonic possession. His only hope? He must dredge up his repressed childhood memory of being raped (just after eating red licorice candy) by a man who pretended to be yes, folks a library policeman. Finally, after having relived this trauma, Sam can battle Ardelia in a gruesome showdown, using a big ball of red licorice as his sticky secret weapon.
The two shorter novellas are a lot easier to take. ''Secret Window, Secret Garden'' returns to King's preoccupation with being a writer: the psychic strain, the price of fame. Like King, best-selling novelist Mort Rainey is more popular than celebrated: Critics say he's ''not really an original writer.'' But Mort doesn't mind being called derivative until a menacing hillbilly named John Shooter shows up at Mort's lakeside retreat, accusing the famous author of stealing a short story. Mort knows he's innocent of plagiarism yet can't prove it. Shooter, furious that Mort won't 'fess up, gets violent. So Mort's panic escalates nicely. It's only too bad that all this zesty suspense leads, eventually, to one of King's most gimmicky, least convincing finales.
''The Sun Dog,'' the collection's simplest, most distinctive story, begins when Kevin Delevan, of Castle Rock, Maine, receives just what he wanted for his 15th birthday: a Polaroid Sun 660 camera. There's only one problem. No matter what Kevin takes a picture of, the print that pops out shows a large black dog. One other thing: With each photo, the dog gets larger. . . and closer. . . and meaner, as if he's about to leap out at somebody's throat.
How exactly did this demonic dog get inside Kevin's camera? Wisely, King never gets too specific. Instead, he turns much of ''The Sun Dog'' into a delicious black comedy, as slimy Pop Merrill, Castle Rock's leading loan shark, steals the camera and tries to sell it for big bucks to some of the area's rich, occult-loving eccentrics. At the same time, the story works neatly as an offbeat coming-of-age adventure for Kevin, who must not only outwit Pop Merrill but also fend off the imminent dog attack. And, throughout, King shows the yarn-spinning command, the ease, that's missing elsewhere in this book: a certain best-seller, of course, but the work of a surprisingly uncertain craftsman. C+