What does " in accounts of" mean in this context? And i don't understand the sentence
... of which one reads in accounts of former visitations of this sort.
Paraphrase
... of which one reads in stories about former visitations of this sort.
account /əˈkaʊnt/ noun
- a report or description of an event or experience: "a detailed account of what has been achieved" synonyms description, report, version, story, narration, ... more
Oxford Dictionaries https://uk.search.yahoo.com/search?fr=mcafee&type=E211GB0G0&p=account+definition
Paraphrase of the whole sentence
He even told Rieux that it was really a great improvement on the deathcarts driven by Negroes of which one reads in accounts of former visitations of this sort.
He even told Rieux that it (i.e. something mentioned previously) was really a great improvement on the deathcarts (which were driven by Negroes) that you can read about in stories concerning visitations (presumably occurrences of plague) of this sort.
i.e. you can read about the deathcarts.