any novel in which his romance tradition is continuing in 20th century?
"The Once and Future King", "Arthur Rex", and "The Mists of Avalon".
Modern legend
In the latter half of the 20th century, the influence of the romance tradition of Arthur continued, through novels such as T. H. White's "The Once and Future King" (1958), Thomas Berger's tragicomic "Arthur Rex" and Marion Zimmer Bradley's "The Mists of Avalon" (1982)
when did the arthurian legends reach their height of popularity With the outbreak of the Second World War. who was he? He was a legendary British leader who, according to medieval histories and romances, led the defence of Britain against Saxon invaders in the late 5th and early 6th centuries. who was he married to? UNANSWERABLE
Yes, through "The Once and Future King", "Arthur Rex", "The Mists of Avalon" etc. Novels such as T. H. White's "The Once and Future King" (1958), Thomas Berger's tragicomic "Arthur Rex" and Marion Zimmer Bradley's "The Mists of Avalon" (1982) in addition to comic strips such as "Prince Valiant". "Arthur of romance"
King Arthur King Arthur King Arthur
Modern legend Modern legend Tennyson and the revival
In the latter half of the 20th century, the influence of the romance tradition of Arthur continued, through novels such as T. H. White's "The Once and Future King" (1958), Thomas Berger's tragicomic "Arthur Rex" and Marion Zimmer Bradley's "The Mists of Avalon" (1982) in addition to comic strips such as "Prince Valiant" (from 1937 onward).  In the latter half of the 20th century, the influence of the romance tradition of Arthur continued, through novels such as T. H. White's "The Once and Future King" (1958), Thomas Berger's tragicomic "Arthur Rex" and Marion Zimmer Bradley's "The Mists of Avalon" (1982) in addition to comic strips such as "Prince Valiant" (from 1937 onward). Tennyson had reworked the romance tales of Arthur to suit and comment upon the issues of his day, and the same is often the case with modern treatments too. Bradley's tale, for example, takes a feminist approach to Arthur and his legend, in contrast to the narratives of Arthur found in medieval materials, and American authors often rework the story of Arthur to be more consistent with values such as equality and democracy. In John Cowper Powys's "" (1951), set in Wales in 499, just prior to the Saxon invasion, Arthur, the Emperor of Britain, is only a minor character, whereas Myrddin (Merlin) and Nineue, Tennyson's Vivien, are major figures. Myrddin' This interest in the "Arthur of romance" and his associated stories continued through the 19th century and into the 20th, and influenced poets such as William Morris and Pre-Raphaelite artists including Edward Burne-Jones. 
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King Arthur [SEP] Modern legend
In the latter half of the 20th century, the influence of the romance tradition of Arthur continued, through novels such as T. H. White's "The Once and Future King" (1958), Thomas Berger's tragicomic "Arthur Rex" and Marion Zimmer Bradley's "The Mists of Avalon" (1982) in addition to comic strips such as "Prince Valiant" (from 1937 onward). Tennyson had reworked the romance tales of Arthur to suit and comment upon the issues of his day, and the same is often the case with modern treatments too. Bradley's tale, for example, takes a feminist approach to Arthur and his legend, in contrast to the narratives of Arthur found in medieval materials, and American authors often rework the story of Arthur to be more consistent with values such as equality and democracy.