Cheri and The Last of Cheri, by Colette (Ballantine, $2.50). Much more than the story of a gigolo, these two novels constitute Colette's masterpiece, a profound examination of love, aging, and sexuality. Ballantine should be congratulated for making them available in mass-market paperback, and ashamed of failing to name the translator.

The Middle Age of Mrs. Eliot, by Angus Wilson (Penguin, $6.95). In fiction there is a drive toward unity and compactness, every seemingly loose end ultimately woven into a thick braid of meaning. But in life, things simply happen--and we feel as helpless as any ancient Greek before the buffetings of fate. So it is with Meg Eliot, happy and serene in her love of a successful husband--who is one day killed quite accidentally by an assassin's gun shot. His death thrusts her into a life of relative poverty in which she must learn to refashion a world for herself. This is one of Sir Angus Wilson's finest novels, and that is very fine indeed.

Democracy: An American Novel, by Henry Adams; introduction by Noel Perrin (Harmony, $4.95). "The best political novel yet written in America," Noel Perrin wrote of Democracy (1880) in his "Rediscoveries" column in these pages last summer. Now that column has borne fruit in the form of this handsome, readable reprint. NONFICTION

Marcel Proust, by Roger Shattuck (Princeton, $5.95). Now that summer's ended and you still haven't read those three fat volumes of the new translation of Remembrance of Things Past, perhaps what you need is a good guide. After all, those winter evenings can be long. No better Baedeker can be found than this introduction to Proust by the distinguished University of Virginia scholar. Gracefully written, and concentrating on the structure of the timeless, time-haunted masterpiece, it has received both the National Book Award (1975) and general acclaim. Eager readers should also look for Howard Moss' equally insightful and elegant The Magic Lantern of Marcel Proust, reissued by Godine ($4.95).

Bad Blood: The Tuskegee Syphilis Experiment, by James H. Jones (Free Press, $7.95). From 1932 to 1972, more than 400 black Alabama sharecroppers and laborers were the subjects of a government study of the effects of untreated syphilis. This case study in medical racism was called "fair-minded" and "profoundly troubling" by Book World's reviewer.

A Primer of Fly-Fishing by Roderick Haig-Brown (University of Washington Press, $8.95) and Flies: Their Origin, Natural History, Tying, Hooks, Patterns and Selections of Dry and Wet Flies, Nymphs, Streamers, Salmon Flies for Fresh and Salt Water in North America and the British Isles, Including A Dictionary of 2200 Patterns, by J. Edson Leonard (A.S. Barnes, $14.95). Together these two books should sustain the fly fisherman for years. Haig-Brown's book, published nearly 20 years ago, is still a classic full of good advice and shrewd insight. Once the fisherman has exercised the basics as presented by Haig-Brown, this second book with its encyclopedic approach to the entymology of fishing, offers limitless advice on fly-tying.

Pioneer Women: Voices From the Kansas Frontier, by Joanna L. Stratton (Touchstone, $8.95). In the winter of 1975 Joanna Stratton found in her grandmother's attic in Topeka more than 800 memoirs by women who helped settle Kansas. In the 1920s, Stratton's great-grandmother had collected and preserved their remarkable, gritty tales, and Stratton has completed her project in this book, published last year to enthusiastic reviews. MUSIC

Berlioz and His Century: An Introduction to the Age of Romanticism, by Jacques Barzun (University of Chicago/Phoenix, $8.95). This abridgment of the standard Berlioz and the Romantic Century, focuses on the life, leaving aside somewhat Berlioz's time and works. That life is rich enough for many men, the composer of the Symphonie Fantastique being the exact counterpart in music to Delacroix in painting and Hugo in poetry, all three committed to the violent Romantic exaltation of sexual desire, revolution, and Faustian genius.

I Really Should Be Practicing, by Gary Graffman (Avon/Discus, $4.95). It's really not fair that someone who plays the piano so well should also be a gifted writer. But so Graffman turns out to be in his delightful memoirs of the practice room and concert hall. The anecdotes about such legends as George Szell, Horowitz, Toscanini, and Van Cliburn are alone worth the price of this autobiographical divertissement, appropriately subtitled "Reflections on the Pleasures and Perils of Playing the Piano in Public."

The Composer's Advocate: A Radical Orthodoxy for Musicians, by Erich Leinsdorf (Yale, $7.95). One of our finest, and most fastidious, conductors here offers his thoughts on the nature of his profession. Witty and often iconoclastic, Leinsdorf calls young conductors to a more profound study of the score, the musical tradition and the composer's purpose. Beyond its polemical aspect, the book offers the general reader rare insight into how a conductor thinks and interprets a composition.

The Operas of Mozart, by William Mann (Oxford/Galaxy, $14.95). Mann, principal music critic of The Times of London here discusses, in 600 detailed pages, all the theatrical musical works of Mozart. Nearly half the book is devoted to compositions before Idomeneo, though the four great operas -- Le Nozze di Figaro, Don Giovanni, CosMi Fan Tutte, and Die Zauberflote -- each receive between 50 and 100 pages of commentary. A valuable reference for the Mozartian or opera fan.