What is the equivalent of the laboratory notebook in the humanities?
I asked my wife, which works in art history/egyptology. So first, "it depends of how the person works" ;). 

However, it seems that many people are making a large number of thematic reading synthesis, research notes (as we do) and she also pointed out the importance of her personal database, where she stores archaeological artefacts, their descriptions, the related bibliography, personal notes about them, their relations, etc. According to her, this is THE most important thing for her work.