In "The Fall of the House of Usher," what were some of the themes that were explored in the novels that the narrator chose to read?

It is important to note that during the middle section of
this excellent short story, the activities that Roderick Usher engages in all reflect
his disturbed inner-nature and his troubled psyche. He produces paintings that are so
abstract that the narrator is hardly able to describe them. He sings a song that,
although it starts pleasantly enough, eventually brings in evil spirits that destroy the
idyllic setting created by the lyrics and are clearly representative of Roderick's inner
troubles and strivings. His choice of reading likewise represents his own character, as
the narrator says:

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Our books--the books which, for years, had
formed no small portion of the mental existence of the invalid--were, as might be
supposed, in strict keeping with this character of
phantasm.

Thus Roderick
devotes himself to various arcane tomes concerning such topics as forgotten religions
and journeys of exploration, poring over abstract and recondite pieces of information
that perhaps represent his own desire to work out what is happening to him and his
sister. Thus the choice of reading material perhaps represents Roderick's own desire to
make sense of his own situation and malady, and understand how and why the curse that he
refers to has been brought down on him.