An interesting read on the subject, although not exclusively focused on multi-tasking, mostly US-centric and at times a bit wordy, is "The Organized Mind" (https://amzn.com/0147516315) by Daniel J Levin, a professor in Psychology.

Among other things, he explains why multitasking seems rewarding: it "creates a dopamine-addiction feedback loop, effectively rewarding the brain for losing focus and for constantly searching for external stimulation".