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He was born Michael Sadler, though upon beginning to publish novels he altered the spelling of his name to differentiate himself from his father, Michael Ernest Sadler, a historian and Vice-Chancellor of the University of Leeds. The name change was precipitated by the scandal that followed the publication of Sadleir's novel Fanny by Gaslight, with some of the ensuing abuse being mistakenly directed at Sadleir's father.
Before the First World War, Sadleir and his father were keen collectors of contemporary art, and purchased works by young English artists such as Stanley Spencer and Mark Gertler. They were amongst the first collectors (and certainly the first English collectors) of the paintings of the Russian-born German Expressionist artist Wassily Kandinsky. In 1913, both Sadleir and his father travelled to Germany to meet Kandinsky in Munich. This visit led to Sadleir translating into English Kandinsky's seminal written work on expressionism, Concerning the Spiritual In Art in 1914. This was one of the first coherent arguments for abstract art in the English language and its effects were profound. Extracts from it were published in the Vorticist literary magazine BLAST in 1914, and it had a major impact on the development of abstract art in Britain and North America right up until the 1960s. Sadleir's translation is still in print, and it remains one of the most commonly used versions of Kandinsky's book in the English language.
Sadleir was director of the publishing firm of Constable & Robinson, a British delegate to the Paris Peace Conference, 1919, a member of the secretariat of the League of Nations, a novelist, a bibliographer and book collector; Together with Ian Fleming and others, Sadleir was a director and contributor to The Book Handbook, later renamed The Book Collector, published by Queen Anne Press. As collector and literary historian, he specialized in 19th century English fiction, notably the work of Anthony Trollope. He also conducted research on Gothic fiction and discovered rare original editions of the Northanger Horrid Novels mentioned in the novel Northanger Abbey by Jane Austen. Beforehand, some of these books, with their lurid titles, were thought to be figments of Austen's imagination. Sadleir and Montague Summers demonstrated that they did really exist. He was President of the Bibliographical Society from 1944 to 1946.
His best known novel is Fanny by Gaslight (1940), filmed in 1944, a fictional exploration of prostitution in Victorian London. A later novel, Forlorn Sunset, further explored the characters of the Victorian Underworld. His writings also include a biography of his father.
Sadleir's remarkable collection of Victorian fiction, now at the UCLA Department of Special Collections, is the subject of a catalogue published in 1951. His collection of Gothic fiction is at the Albert and Shirley Small Special Collections Library at the University of Virginia.
- Fanny by Gaslight - 1944 (US title: Man of Evil)
- "Derek Hudson, ‘Sadleir, Michael Thomas Harvey (1888–1957)’, rev. Sayoni Basu, Oxford Dictionary of National Biography, Oxford University Press, 2004 (subscriber access only)". Retrieved 2008-05-09.
- Tom Steele, Alfred Orage and the Leeds Arts Club (1893-1923) (Aldershot, Ashgate 1990) 179
-  Archived August 4, 2009 at the Wayback Machine
- Excursions in Victorian Bibliography - London: Chaundy & Cox, 1922
- Trollope: a commentary - 1927
- Trollope: a bibliography - 1928
- Bulwer: a panorama - 1931
- Fanny by Gaslight (Appleton-Century) - 1940
- Forlorn Sunset - 1947
- XIX Century Fiction: a bibliographical record (Constable & Co. and University of California Press) - 1951
- Online text of a brief autobiography, PASSAGES FROM THE AUTOBIOGRAPHY OF A BIBLIOMANIAC, first published in 1951 as the introduction to his XIX Century Fiction: a bibliographical record, and reprinted separately in 1962.
- Sadleir MSS and Sadleir MSS III brief descriptions of manuscripts at the Lilly Library, Indiana University
- Wishful Thinking gravestones of Michael Sadleir and of a relative, Michael Thomas Sadler (born 1916, died 1942)
- Michael Sadleir papers, MSS 2053 at L. Tom Perry Special Collections, Brigham Young University
- Works by Michael Sadleir at Project Gutenberg
- Works by or about Michael Sadleir at Internet Archive
- Works by Michael Sadleir at LibriVox (public domain audiobooks)