The Midnight Folk, by John Masefield (Dell, $4.95; ages 8-up). To most people, John Masefield is merely a Kiplingesque -- and somewhat patronized -- poet, author of "Sea Fever, "Cargoes," and similar salt- water fare. In truth, he was quite a versatile man of letters, author of novels, essays, autobiography, and a pair of children's books, The Midnight Folk and its sequel The Box of Delights. For some years, I had been longing to read The Midnight Folk, partly because of its spooky title, and primarily because it has a reputation as a superb fantasy.

So it was with eagerness that I opened this new edition, in Dell's Yearling Classic series (with an afterword by Madeleine L'Engle). The story tells of the adventures of young Kay Harker, who searches for the treasure of his great grandfather with te help of various animal friends. Old Captain Harker, commander of The Plunderer, had taken the gold vessels and statuary of the Church of Santa Barbara into his protection, but lost the treasure to his own mutinous crew. Now, three generations later, Kay finds that a coven of local witches still seeks the glittering prize -- and they are in league with the evil grandson of one of the original villains.

Most of Kay's adventures take place after dark -- Masefield keeps the reader wondering whether what happens is dream or reality -- and involve a cornucopia of fantasy devices: speaking portraits, flying horses, seven-league boots, time travel, witches' ceremonies, talking animals, living puppets and dolls, King Arthur and his court, mysterious books, invisibility. Many of Kay's individual adventures prove exciting -- especially, his discovery of the witches in the night kitchen and their talking broomsticks -- but the novel begins to grow tiresome after a while. The action is complex, even recurive, with constant shifts back and forth between the past and present, the real and fantastic. The prose sparkles only when Masefield adopts a dialect, as in Kay's exchanges with a wonderful cockney rat and a crotchety, champagne-drinking grande dame. Too often, though, the reader is not sure where he's going and senses that Masefield is simply tacking on adventures as they pop into his mind.

No one will regret reading The Midnight Folk, and for the right child it could become a favorite. But it will be a while before I go on to The Box of Delights.

Workboats, written and illustrated by Jan Adkins (Scribners, $11.95; ages 5-up). Everyone admires craftsmanship, but Jan Adkins both admires and practices it. He has written about hand tools, heavy equipment, the construction of sand castles, and more. His novel A Storm Without Rain dealt largely with boat building (and friendship and time travel). In whatever he undertakes, Adkins demonstrates the care, attention to detail, and the esthetic sense of a man who values hand-work. This latest book depicts the dozens of small and middle-sized boats that ply the coastal waters in Massachusetts -- skiff, dory, yard boat, tug, coast guard cutter -- and their work: ferrying, lobstering, fishing, clamming. The drawings detail the distinctive characters of these vessels, without sacrificing their down-to-earth romance. Adkin's story line -- the search for a missing scallop fisherman -- gives this picture book just enough drama to keep the reader turning the pages instead of spending all afternoon on the pictures.

The Thirty-Nine Steps, by John Buchan; with illustrations by Philip Hood (Jonathan Cape/Merrimack, $18.95; ages 8-up). When adventure novels start to seem dated to adults they are often just beginning their life as children's literature. Think of Treasure Island, King Solomon's Mines or the scientific romances of H.G. Wells. That process seems to be working now on John Buchan's classic pursuit thriller, The Thirty-Nine Steps. The old-fashioned straightforwardness of Buchan strikes a naive, jingoist note for readers used to le Carr,e or McCarry, but for a teen-ager it makes splendid escape fare.

A mysterious stranger tells Richard Hannay of a spy plot in pre-World War I England. That night Hannay returns to his flat, finds his informant dead with a knife in his heart, and the authorities on the way. Rather than risk arrest he takes to his heels -- and the real adventures begin, with both the police and foreign agents in hot pursuit as our hero tries to forestall the sinister plot. Lively action, a terrific evocation of the English countryside, and a textbook example of The Great Game. Philip Hood's period illustrations, especially his depiction of Hannay, recall Alfred Hitchcock's celebrated film.

Piping Down the Valleys Wild, by Nancy Larrick; illustrations by Ellen Raskin (Delacorte, $14.95; all ages); Talking to the Sun, selected by Kenneth Koch and Kate Farrell (Museum of Modern Art/Holt, Rinehart and Winston, $18.45; all ages); Doodlesoup, by John Ciardi; illustrated by Merle Nacht (Houghton Mifflin, $11.95; all ages); My Parents Think I'm Sleeping, by Jack Prelutsky; pictures by Yossi Abolafia (Greenwillow, $10.25). Preschoolers love rhymes, young children favor nonsense verse and riddles, and teen-agers scorn poetry as smarmy -- until the urge comes to write a sonnet to the beloved, impossible object in the next home room. Along with Walter de la Mare's Come Hither and Geoffrey Grigson's The Cherry Tree, Nancy Larrick's 1968 collection -- newly reissued -- has come to rank as a classic. In her selections, Larrick prefers 20th-century verse, and favors poets who write specifically for young people (Milne, Merriam, McCord, Farjeon, Kuskin, etc.), but there are also lyrics from Blake, Shakespeare and Browning, and even examples ofSwinburne, Yeats, Williams and Eliot. A favorite since its first publication.

The Koch-Farrell anthology reflects current American tastes -- lots of work by contemporary poets, from Frank O'Hara to L,eopold Senghor, and a fair amount of folk verse from Africa, India, China, and pre-Columbian America. Every page carries illustrations drawn from the holdings of the Museum of Modern Art. The resulting marriage of painting and poetry makes this a handsome album, albeit one whose artsiness may put off some young readers.

Any man whose work includes an acclaimed translation of Dante and a mastery of light verse is one to reckon with. John Ciardi's new volume of children's poetry is as bouncy and witty and fun to read as any going: "There was a man from nowhere/ He never got where he went./ He bought a ticket out of there./ It cost him what he spent. . . " Another master of verse for kids, Jack Prelutsky, groups his latest work around sleep and bedtime. "Tonight is impossibly noisy,/ it's filled with a horrible sound,/ as if dozens of ogres and tigers/ are stuck on a merry-go-round./ It sounds like a monkey battalion/ is dancing on needles and pins,/ or an out-of-tune elephant orchestra/ is sawing on steel violins." Two more stanzas follow of equal verve and zippiness.

Heroes, Monsters and Other Worlds from Russian Mythology, text by Elizabeth Warner; illustrations by Alexander Koshkin (Schocken, $15.95; ages 7-up). Mythology and folktale -- the great ocean of story -- should be a part of every childhood, and the Schocken "World Mythology" series goes far toward satisfying even the most voracious appetite for narrative. This latest volume retells the classic Russian bylinas (songs of ancient heroes), skazkas (tales of wonder), and bylichkas (peasant stories of encounters with the supernatural). Warner's text draws from the famous collections of Afanas'ev, and includes tales of that perennial favorite, the skeletal witc Baba Yaga, whose house is built of human bones and walks around on chicken legs. Koshkin's pictures -- reminiscent of folk paintings -- dazzle with their animal vitality: wolves and bears leap at the reader's throat; mighty Cossack warriors, dressed in dark brocade and broadsword, ride through misty landscapes; immense forests at twilight loom in the background. Very Russian yes, but universal too.

Solomon, The Rusty Nail, by William Steig (Farrar, Straus and Giroux, $12.95; ages 5-up). William Steig has been producing classic children's books for years -- Abel's Island, CDC?, Doctor De Soto, The Real Thief -- and even his minor efforts, as Solomon, The Rusty Nail must be counted, possess immense charm. Solomon, a young rabbit who dresses like a little boy, discovers that he can turn himself into a rusty nail, a trick that he soon employs in mystifying his family. One day, however, he finds himself pursued by a one-eyed, knife-wielding cat (named Ambrose), and must transform himself to escape capture. Unfortunately, Ambrose notices the trick, takes the nail home, and cages Solomon, to await the moment the rusty metal will once again become juicy plump bunny. Three weeks pass. In frustration, Ambrose nails Solomon to the side of his house, which effectively prevents any metamorphosis. Eventually, though, all ends happily.

Steig's watercolors make this simple tale delightful. Note, for instance, Solomon's wonderful startled look when the vicious cat surprises him while butterfly-hunting. Indeed, all the facial expressions -- of Ambrose's wife Clorinda hanging wash, of Solmon's pipe-smoking dad -- are immensely appealing, and neat examples of Steig's winning artistry.