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Michael Sadleir (25 December 1888 – 13 December 1957) was a British publisher, novelist, book collector and bibliographer.
Michael Sadleir was born in Oxford, the son of Sir Michael Ernest Sadler and Mary Ann Harvey. He later adopted the older variant of his surname to differentiate himself from his father, a historian, educationist, 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.
Sadleir was educated at Rugby School and was a contemporary of Rupert Brooke and Geoffrey Keynes. He then attended Balliol College, Oxford, where he read history and won the 1912 Stanhope prize. 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 began to work for the publishing firm of Constable & Robinson in 1912, becoming a director in 1920, and chairman in 1954. In 1920 as editor of Bliss and Other Stories by Katherine Mansfield for Constable he insisted on censoring sections of her short story Je ne parle pas français which show the cynical attitudes to love and sex of the narrator. Her husband John Middleton Murry persuaded Sadlier to reduce the cuts slightly (Murry and Sadleir had founded the avant-garde quarterly Rhythm in 1912) 
After the end of World War I, he served as a British delegate to the Paris Peace Conference, 1919, and worked at the secretariat of the newly formed League of Nations. As a literary historian, he specialized in 19th century English fiction, notably the work of Anthony Trollope. 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. 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.
Sadleir's best known novel was 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, published in 1949, and a privately published memoir of one of his sons, who was killed in World War II.
The remarkable collection compiled by Sadleir 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.
- "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.
- Michael Sadleir Papers, 1797-1958, unc.edu. Retrieved 15 July 2017.
- Tom Steele, Alfred Orage and the Leeds Arts Club (1893-1923) (Aldershot, Ashgate 1990) 179
- Alpers (editor), Antony (1984). The Stories of Katherine Mansfield. Auckland: Oxford University Press. pp. 551, 560. ISBN 0-19-558113-X.CS1 maint: Extra text: authors list (link)
- Sadleir, Michael (1927). A Footnote to Jane Austen. Oxford: OUP.
- The Bibliographical Society -- Past Presidents Archived 2009-08-04 at the Wayback Machine, bibsoc.org.uk (archived webpage). Retrieved 15 July 2017.
- Excursions in Victorian Bibliography - London: Chaundy & Cox, 1922
- Desolate Splendour - 1923
- Trollope: A Commentary - 1927
- Trollope: A Bibliography - 1928
- Evolution of Publishers' Binding Styles - 1930
- Bulwer: A Panorama - 1931
- These Foolish Things - 1937
- Fanny by Gaslight - Appleton-Century, 1940
- Things Past 1944
- 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.
- "Nineteenth Century Literature". UCLA Library Research Guides. University of California, Los Angeles. Retrieved 7 April 2017. More than 4600 titles mainly from the 19th century including important novelists, series, and cheaply published yellowbacks.
- The Sadleir-Black Collection of Gothic Fiction, Albert and Shirley Small Special Collections Library at the University of Virginia. collection of Gothic fiction titles assembled by Sadleir, Arthur Hutchinson and Robert Kerr Black.
- Michael Sadleir Papers, 1797-1958 description of archival material held in the Wilson Library at the University of North Carolina at Chapel Hill.
- Michael Sadleir papers, MSS 2053 at L. Tom Perry Special Collections, Brigham Young University
- Sadleir MSS and Sadleir MSS III brief descriptions of manuscripts at the Lilly Library, Indiana 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)