Books like "Salt" and "Opium" tended more toward a macro view of the history of how the substance affected civilizations and history. "Cannabis" was more personalized. It spent a lot of time on how cannabis has affected literature and specific people. It also concentrates on the 20th century, especially the 60's (not surprisingly.)"Cannabis" also makes a strong case for legalization/decriminalization. There is a lot of information on the politics around the drug and its other uses (e.g. fiber). The author cites several studies showing that marijuana is not harmful. Only one study showing concerns about decriminalization was mentioned and that study was used to back the author's views on a broader subject.
I prefer the macro view which is why I gave it 3 stars. The author obviously had an agenda which the book was written to support. That is certainly his right and privilege, but I would have like a more balanced view.