Google's NotebookLM builds a private notebook from the documents you upload - papers, PDFs, notes, reports - and answers only from those, with citations back to the passage. Because it does not draw on outside information, most of the hallucination risk that comes from a model's general knowledge is removed.
Strongest at: closed-corpus analysis. Interview transcripts, a fixed set of papers or internal reports can be questioned directly, with every answer traceable to its source.
Worth knowing: if the answer is not in what you uploaded, it will not help. That is the point, and also the limit.
Best for: qualitative research, dissertation reading and internal knowledge bases.