A self-taught scholar with a day job, he is best known for his The Art of Accompaniment from a Thorough-Bass (1931), described as the finest piece of musicography ever produced in England.
Performers were expected to improvise (realize) these to suit the occasion, guided by no more than a bare sketch called a figured bass (or thorough-bass).
But the rules of Baroque harmony were quite strict, and when he tried to play he found he was running into many problems, technical and artistic, to which modern textbooks seemed to supply no answers.
[12] Arnold's approach was to recover the old learning, insofar as it was in writing and had survived, and to compile a synthesis of this knowledge illustrated by many practical examples.
[3] Describing his collection in his book (1931) Arnold himself said: The present treatise represents the labour of many years, and its compilation would have been impossible had not the writer been fortunate enough to acquire gradually all but a very few of the works (both didactic and musical) to which reference is made in its pages.
He has pleasant memories of a little shop in St Martin's Lane, long since closed, where second-hand music was sold, among which treasures were sometimes to be found...[13] Like a true bibliophile, Arnold not only spent freely in acquiring his books (and if a better copy of a work already in his possession was brought to his notice he often purchased it), but his loving care for his treasures prompted him to have them exquisitely bound, often in full leather with gilt tooling.
[14] In 1931, despite the Depression and a thin subscription list, the Oxford University Press published his The Art of Accompaniment from a Thorough-Bass as Practised in the XVIIth and XVIIIth Centuries.
[15] Its first sentence reads:The first object of the present work is to give adequate information concerning the way in which the accompaniment founded on a Basso continuo, or Thorough-Bass, was actually treated during the period, extending over wellnigh two centuries [i.e. the Baroque era], when such an accompaniment was, with few exceptions, a necessary part of every musical performance, solo or concerted, vocal or instrumental.His 'adequate' information was exhaustive: the book's 918 pages cited the works of 110 Baroque practitioners, well known and obscure, with a wealth of practical examples.
Morris (then professor of counterpoint at the Royal College of Music) it seemed "certain to stand for all human time as the standard work of reference".
[25] Although Arnold published other works – he wrote the article on Thorough-bass for Grove's Dictionary of Music and Musicians – his masterpiece was his book.