Frank Kogan

Joan Aiken: "The Whispering Mountain" -- a scholarly boy (grandson of the hated museum caretaker) joins forces with the local unschooled lads and a gypsy girl (and her poet father) to defeat the knavery of a wicked aristocrat -- dido twite doesn;t actually feature in this sadly, tho characters do that will later meet her in other books (i think)

Leon Garfield: "Smith" -- an urchin pickpocket nabs an important document moments before his victim is knifed by sinister grown-up villains; he can't read; his idol is a highwayman -- set in early 18th century london

Alan Garner: "Elidor" -- kind of a kitchen sink version of a narnia story, set in late 50s manchester

Richard Hughes: "A High Wind in Jamaica" -- but you already read this

Tove Jansson: the late moomin books, as already advised (tho all her books are grebt); but also "the summer book", about a small child and a crabby granny on a tiny island in the finnish archipelago

William Mayne: "sand" if you can find it, about schoolkids in a small north-eastern coastal town being swallowed by the dunes; "earthfasts", about an 18th century drummerboy who falls through a timeslip into modern england, and what befalls him (also its sequels "cradlefasts" and "candlefasts"); he wrote for younger ("no more school" is terrific) but no books for grown-ups

"The Giant Under the Snow" and "The House on t5he Brink" are two terrific horror stories for teens by John Gordon

"A Hundred Million Francs" is a tremendous "gang of kids foils some crims" tale by Paul Berna

I am enomously fond of Erich Kästner, but he isn't exactly "my era"