Testimony lies at the intersection of epistemology and ethics, and it is clear why this must be so. To evaluate testimony as credible involves evaluating the source of testimony, the person giving it, as credible, at least for the particular purposes in question. Thus acceptance of testimony involves evaluation of the testifier's character. And any evaluation of a testifier's character is capable of being just or unjust.
Thus there arises the problem of testimonial injustice, which occurs when our evaluation of testimony involves injustice against the testifier. A fairly straightforward example is when a black man's testimony is given less weight than a white man's testimony simply because he is black. Such an evaluation of testimony involves injustice against the black man, because it involves disrespecting him as a collaborator in the search for truth. Cases like this has led some people to conclude that testimonial injustice can only exist where there is a pre-existing stereotype or prejudice, but I think this is false for the same reason it is false that other forms of injustice require a pre-existing stereotype. These spur-of-the-moment lapses in justice (e.g., due to one's mood at the time) are still injustices, and it is clear that, for instance, anger at the way someone has expressed his testimony can lead to unjust discounting of his credibility as a testifier in this case, involving the same sort of disrespect for them as a rational creature like oneself.
Thus testimonial injustice provides a fascinating example of the way justice is relevant to the pursuit of knowledge.
Miranda Fricker had an interesting interview on the subject of testimonial injustice not long ago. OUP allows you to look at the introduction (PDF) to her recent book on the subject, Epistemic Injustice: Power and the Ethics of Knowing; she has an interesting discussion of To Kill a Mockingbird from this angle.