"Users’ expectations are different for these two. When you search books you expect an answer; when you search journal articles, a scholar expects a list of things to read. A book represents late-stage work, not the early-stage work of journal articles."
Thus Anurag Acharya, one of the founders of Google Scholar, on different kinds of reading, at the ALPSP Annual Conference in October 2015 (a presentation helpfully summarised by John Sacks in a Scholarly Kitchen post).
“You can easily see the difference between these two modalities. Do a search in Google for “San Francisco weather” and the answer pops up: the current temperature and conditions, and the forecast. But for the “weather scholar” there is also a list of sites below the forecast that you can go to if you want to study the topic by reading web pages about San Francisco weather.”