Much of the theory is a little hoary, like Douglass's "A+B theorem" which appears to overlook multiplier effects. But some of it appears of contemporary relevance. For example, he emphasizes the stock of knowledge and technique and wealth. From the Wikipedia article:
And he was correct in focusing on the nature of money:
Douglas disagreed with classical economists who divided the factors of production into only land, labour and capital. While Douglas did not deny these factors in production, he believed the “cultural inheritance of society” was the primary factor. Cultural inheritance is defined as the knowledge, technique and processes that have been handed down to us incrementally from the origins of civilization. Consequently, mankind does not have to keep “reinventing the wheel”. “We are merely the administrators of that cultural inheritance, and to that extent the cultural inheritance is the property of all of us, without exception.” Adam Smith, David Ricardo and Karl Marx claimed that labour creates all value. While Douglas did not deny that all costs are ultimately due to labour charges of some sort (past or present), he denied that the present labour of the world creates all wealth.
He was also very conscious of the issue of abundance, which I am very interested in. Scarcity is no longer the essential problem of mankind, but distribution (and incentivizing innovation and good behavior).
According to economists, money is a medium of exchange. Douglas argued that this may have once been the case when the majority of wealth was produced by individuals who subsequently exchanged it with each other. But in modern economies, division of labour splits production into multiple processes, and wealth is produced by people working in association with each other. For instance, an automobile worker does not produce any wealth (i.e., the automobile) by himself, but only in conjunction with other auto workers, the producers of roads, gasoline, insurance, etc. In this view, wealth is a pool upon which people can draw, and money becomes a ticketing system. The efficiency gained by individuals cooperating in the productive process was coined by Douglas as the “unearned increment of association” – historic accumulations of which constitute what Douglas called the cultural heritage. The means of drawing upon this pool is money distributed by the banking system.
Douglas believed that money should not be regarded as a commodity but rather as a ticket, a means of distribution of production.
He proposed a "national dividend" to consumers, a form of basic income, to make up for inherent deficiencies in effective demand.
Douglas also claimed the problem of production, or scarcity, had long been solved. The new problem was one of distribution. However; so long as orthodox economics makes scarcity a value, banks will continue to believe that they are creating value for the money they produce by making it scarce. Douglas criticized the banking system on two counts:
for being a form of government which has been centralizing its power for centuries, and
for claiming ownership of the money they create.
The former Douglas identified as being anti-social in policy. The latter he claimed was equivalent to claiming ownership of the nation. According to Douglas, money is merely an abstract representation of the real credit of the community, which is the ability of the community to deliver goods and services, when and where they are required.
I don't think the solutions he proposes necessarily work out, although I haven't read the original books and papers. But much of the diagnosis is intriguingly correct.