The One Thing by Gary Keller
I like the premise, though I'm not convinced it requires an entire book to convince the reader that 'multitasking' is not effective. The book does offer good practical advice for blocking time out on the calendar and puts an emphasis on always focusing on just one task.
To be honest, I've struggled with trying to manage too many tasks at once and really try to put effort into ensuring my goal at all times is clear, achievable, and has a reasonable stopping point.
Read in 2017.