FOR SOME REASON, people always ask novelists how long it took them to write their books, as if a novel which took nine years to produce is somehow more valuable then one which took nine months. Dick Francis' work proves this measure invalid, because he's given us 20 mighty fine books in as many years. Despite the fact that he produces like a factory, none of his books smacks of the assembly line, nor has anyone ever hinted that he writes too speedily for serious consideration, by which I mean notice in the heavy-duty press: The New Yorker, Harper's, Saturday REVIEW, and other national outlets.

Dick Francis started getting that kind of critical attention -- what we in the business call being "well-reviewed" -- in 1964, when Nerve, his second novel, appeared in this country. In his native England, he was already so famous that that his first book, in 1957, was not a novel at all but an autobiography, The Sport of Queens, which detailed his spectacular career as a steelechase jockey.

Francis reckoned then that he'd be known as "the man who didn't win the National," referring to the great Grand National steeplechase at Aintree. Instead, though, he's garnered Britain's Silver Dagger (in 1965, for For Kicks) and an Edgar (for Forfeit in 1969). He's also won acclaim sufficiently wide to have sent his latest book, Reflex, back for a second printing before its official publication date.

Amazing, then (amusing, too) that Publishers Weekly, which ought to know, is touting this as Francis' "breakthrough novel," separating it, in fact, from what it terms his "racing mysteries." Reflex is up to its hocks in the horsey world that Francis knows and delineates so very well. Indeed, one fairly thick strang in Reflex's tightly braided storyline is jockey Philip Nore's reluctance to bid the sporting life adieu. What interests us in that life -- even if we wouldn't know the difference between a whinny and a moo -- is that we see it from within. This first-person point of view if typical of Francis and typically, too, we're tossed right into the rough-and-tumble, from the opening sentence on: "Winded and coughing, I lay on one elbow and spat out a mouthful of grass and mud. The horse I'd been riding raised its weight off my ankle . . . " To say "typical" is thus to give the book high praise. We're hooked, and with no wasted motion, no lather. Dick Francis' prose is, as always, spare.

And yet replete. When he has Philip Nore describe someone, we never feel the want of any detail. Consider Jeremy, who becomes Nore's friend -- yet another strand of plot: "He was about twenty-five, tall as a stork and earnest, with office-colored skin. Charcoal flannel suit, striped tie, no binoculars, and no air of belonging where he stood, in the business-only section of the racecourse." We'd know Jeremy anywhere, he's that distinctly drawn.

But it isn't Francis' lean and supple style alone that makes this book. It's pacing, too. Weight, for instance, how we come to know Nore. A loner when the story opens, Nore nonetheless reveals himself to us, bit by tantalizing bit. It stirs more than our curiosity when he says, "One thing may haphazard upbringing had given me was an almost limitless capacity for waiting. Waiting for people to come, who didn't; and for promises to be fulfilled, that weren't." It will be some while before Nore elaborates on the circumstances hinted at here. Another hook. Two short sentences dropped less than 10 pages into the book and we want to know more, we want to know everything, and we care.

Later on, Nore's objectively simple acts will move us incredibly, because we have come to know him so well, know what gestures take from him, how they matter. In other words (though I hate to rattle the gate this way), Francis, whose timing is as good as his choice of words has us right on course.

But Reflex could nose out the author's earlier entries on the basis of its plot alone. Not only do we have the two subplots aforementioned and one other, but a main plot -- the mystery, which is the unraveling of photographic puzzles left behind by a character Nore despises when the book starts -- with three separate elements, any one of which could have carried the book.

And do we ever get confused? Nope. Do we ever feel that any portion doesn't fit, is tacked on, hodge-podged together with the rest? No again. Francis has it all in hand, and he makes it look so easy that it's hard not to accuse him of larking, showing off. Or making one last fleet run before going into retirement (although his publisher insists this is not the case: "Dick Francis is always working on another book," a Putnam's spokeswoman said, even hinting that computers will play the role in his next that photography does in this one). We can only be glad.