WHEN AGATHA CHRISTIE was translated to the great bookstore in the sky, there was much speculation in the publishing world about who would take her place as a mystery best seller. It is now clear that the mantle has fallen on the shoulders of Dick Francis. Proof is his 26th book, and every one of his recent efforts has had a healthy run on the best-seller lists and enormous paperback sales in a spectrum of languages.

Proof will undoubtedly have a comfortable springtime at the top of the charts. All the classic Francis elements are there in abundance. There are the protagonist darkened by personal tragedy, beautifully drawn cameos of English eccentrics, the horse-racing background and enough blood and guts spattered about for several books on surgery. This time, though, there is an added fillip. Tony Beach, the protagonist, owns a wine and liquor store; his expertise as a taster of Scotch and wine is at the heart of the story. Like Arthur Hailey, Francis loves to convey information in his books and does it neatly and almost unobtrusively. Over the years, he has taught his readers nearly everything there is to know about horse-racing. In Proof, there is a small encyclopedia of facts about booze. We learn, for example, that in the year 1795, the harvest in Bordeaux started on Sept. 24 and that the year 1816 produced undrinkable wine. I found all his information to be accurate and his judgment on wine and liquor to be sound. It is evident that not only has Francis done his homework but that he has quaffed a bottle or two in his time.

The gore starts to flow early in Proof, as a 10-ton horse van plows into a tentful of merrymakers, and barely begins to coagulate before Beach and a friend stop a couple of shotgun blasts. Sandwiched in there somewhere is a very nasty murder by plaster of Paris. Such mayhem not only captures the attention of the reader but is also part of the Francis technique. He establishes the beastliness of the villains and the general bloodiness of the world and then threatens to bring it all down on the head of the hero, of whom the reader has grown rather fond. You can't help but begin to fear for Tony Beach's safety -- and Francis has you in his grip. This is genre writing, but Francis is a master of it.

One characteristic of the genre novel is that it more clearly mirrors its times than do more ambitious works of art, which often predict the future or partially transcend their era. Take a look at some of the best-known names in the British mystery-thriller tradition. Agatha Christie, even in her postwar novels, reflects the mores of a simpler small- town England before World War II, seen through the eyes of an upper-class woman. Ian Fleming, writing in the late '50s and '60s, reflects the increasing fear of atomic- weapon-style technology, which equally threatens both West and East. John LeCarr,e reflects the growth of international corporation culture in the '70s. His George Smiley, wrestling with bureaucracy and incompetent or malign bosses, intensely focuses on achieving his appointed goal. He is the manager as hero.

Though some of his novels predate the Iron Maiden's arrival at the center of Britain's political life, I see Dick Francis's stories as a reflection of the era of Margaret Thatcher. There is a spare-the-rod-and-spoil- the-child tone that erupts frequently. In Proof, Beach's friend McGregor says at one point: "It is fashionable to explain away all crime as the result of environment, always putting the blame on someone else, never the actual culprit. No one's born bad, all that sort of thing. If it weren't for poor housing, violent father, unemployment, capitalism, et cetera, et cetera. You've heard it over and over." (The fact is, of course, that no one has heard "it" even once as a full exculpation of violent criminal acts, except from the utterest of fools or, very properly, from lawyers defending their clients.) Since sentiments similar to McGregor's pepper books by Francis, I take them to be the author's own. He sets up a straw liblab and then knocks him down. Rampant Thatcherite grumpiness.

At any rate, Dick Francis wasn't hired to be a professor of philosophy or attorney general. We buy his novels for amusement, for story-telling, for suspense. He seems to get better at his craft all the time, and Proof may well be his best novel yet.