Meetings, most of them without bullet points or a specific topic.
I'm coordinating another devs too and that costs less time then meetings to projects. We don't have the best product owner culture, so I need to fill that role too. But we are working on that topic atm and we are reaching the first goals.
It's funny and I like it, but sometimes I just block time in my calendar to do tasks two weeks ahead. So nobody is able to schedule me on a meeting without asking before.
That makes most people think about if I'm really necessary.
It's not a book, but I loved it and it is really helpful.