"He was a Scottish gentleman with a big beard"

It is likely that Carter was alluding to specific historical figure in mentioning the 'Scottish gentleman'.  Based on chronology, the likelihood of his owning a fine library and the size of his beard, the most likely contender appears to be Thomas Carlyle.

Thomas Carlyle (1795-1881) was a satirical writer, essayist, historian and teacher.  The son of a preacher, he lost his faith at university but retained strong Calvinist principles.  His work appealed to Victorians struggling to make sense of the changing morality and new social order created by scientific advancement and political upheaval.

Carlyle moved to London in 1834 and published his most famous work, The French Revolution: A History in 1837.  Its three volumes were not only a factual history but an exploration of the motivations for action during the Revolution.  He praised individual human action above ideological formulae and called for 'heroes' to direct their spiritual energy into practical action.

Carter's description of Ma Nelson suggests that her political sympathies may have been similar to Carlyle's, and there is little doubt that he owned a large library; he is quoted as saying, "In books lies the soul of the whole Past Time; the articulate audible voice of the Past, when the body and material substance of it has altogether vanished like a dream."  Furthermore, his marriage was known to be unhappy, and it is believed that his wife Jane had an emotional and perhaps physical attachment to his friend Edward Irving.