Normally, you use desktop search to navigate to something you know is on your machine, and use search in a web browser to look for something you don't have.
I mean, if someone searches for a book name in desktop search, it's most likely because they want to open the unlicensed PDF copy they think is on their hard disk, and would thus either open it or download a new one if it's not actually there, rather than buying anything.
It seems Canonical would only make decent income if they either somehow convinced everyone to use desktop search for all web and product searches too, or implemented the feature in Ubuntu's browsers instead of the dash.
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