"Deep Work" by Cal Newport. It completely changed how I view my work, and how I spend my time, more than any other generic self-help book I've come across. It isn't the usual "delete Facebook, and everything will be fine" blog post. The main take-away for me is making a well-defined separation between "busy"/"shallow" work and "deep" work. This takes the form of spending long, uninterrupted periods engaged in deep work (e.g. academic research), with shorter periods allocated for shallow work (e.g. checking and responding to emails). I have managed to allocate one day per week for myself where I completely turn off WiFi, allowing me to focus on whatever task I choose. This means not accepting the temptation to respond to interruptions from instant messaging apps, email, news websites, etc., and it has worked wonders for my productivity! For me, it serves as a weekly reminder of the insane amount of content I consume while online, along with the (highly expensive) constant context-switching I must perform to do this.