This chapter hasn't been written yet.
It will offer a detailed explanation of the HookM monad, which is largely equivalent to HalogenM but with a slightly different approach to state. It will also describe why HookM is used as the action type in Hook components' HTML.
In the meantime, you can see the examples to see this type in action. There are also some tips available in the Hooks API reference.
You may also wish to read the Implementing UseWindowWidth section of the introduction to Halogen Hooks for more information.