Dear N T:

You wrote:
> They were both working on Rousseau’s lesser-known text
> “The Origin of Language”.
>
> Doesn’t this mean that Rousseau had only written two
> texts?
> Shouldn’t it be “one of his lesser-known texts”?
>
Yes, you are correct. Unless there was an established context, the example would suggest that there were only two texts. To qualify it properly, your example is standard.