This is one of the most known books about about OKRs. Unfortunately (or fortunately, depending on your viewpoint) this is mostly geared towards convincing senior management of why they should use OKRs.

But it lacks any details on how to execute the concept, it is mostly a lot of testimony, case studies and some superficial guidelines. But it can be used to convince senior managers on why they should use it, as it is written in very simple language, with short chapters (if they manage to read it).

I recommend that it is read, as it is important to understand why the concept exists, why it has been proven to be successful, and what it can be used for. But you need other, more execution books to be able to implement it efficiently-