02138, a magazine aimed at Harvard alumni, has a great article about the widespread practice among professors of using low-wage student labor to research and even write their books.

...in any number of academic offices at Harvard, the relationship between “author” and researcher(s) is a distinctly gray area. A young economics professor hires seven researchers, none yet in graduate school, several of them pulling 70-hour work-weeks; historians farm out their research to teams of graduate students, who prepare meticulously written memos that are closely assimilated into the finished work; law school professors “write” books that acknowledge dozens of research assistants without specifying their contributions. These days, it is practically the norm for tenured professors to have research and writing squads working on their publications, quietly employed at stages of co-authorship ranging from the non-controversial (photocopying) to more authorial labor, such as significant research on topics central to the final work, to what can only be called ghostwriting.

Ideally, this would constitute a sort of apprentice system — one generation of scholars teaching the next through joint endeavor. But in reality the collaborative element, though quietly sanctioned by universities (the article goes into this a bit), receives no direct blessing or stated pedagogical justification. A ghost ensemble works quietly behind the scenes to keep up the appearance of heroic, individual authorship.