Reddit reviews Shattering The Great Doubt: The Chan Practice Of Huatou

Rinzai Zen uses the "hua-t'ou" method of inquiry for the "first koan" or "preliminary koan." Which in some ways is quite different from other koans. It can be sort of tricky to explain. Many teachers probably prefer not to give too detailed instructions -- filling the student with words and ideas. But you shouldn't approach it as a mantra or a mere object of concentration. For some good advice on what to do with it,

• here is a page from the Zen Buddhist order of Hsu Yun explaining hua-t'ou practice in a friendly way;
  
• and here is a paper by Stuart Lachs going into more detail.
  
  Here is an important point from Lachs's paper:
  
  > One of the important reasons for practicing the hua-t’ou today is that, as lay people, it allows us to practice a good part of our days, rather than just when on the cushion. Its practice really demands that we confront important issues of life, often throughout the day. The practice calls for us to work on the hua-t’ou throughout the day so that the doubt becomes part of our lives, a fertile doubt, at times, seemingly working below our conscious level.
  
  And a nice paragraph that shows the basic point of this kind of practice:
  
  > Perhaps the first aspect to notice is that all hua-t’ou are questions that appear open ended. They seem more to tease or provoke our minds to look deeply into the nature of being and our very own being. Another aspect to notice is that we are the subject of the practice, this body, this mind, not some far away being. Entering into hua-t’ou practice is inviting ourselves to be taunted or perhaps even tormented with a basic question of being and life. We are forcing ourselves into a situation where it is necessary to confront our very being. This is not exclusively a “feel-good” process, it can be very frustrating, frightening at times, disappointing, and boring at times. But it can also be fertile and lead to seeing our true Nature and to directly entering the
  world of Zen.
  
  If you would like something more thorough, there is a book called Shattering the Great Doubt by Sheng Yen.
  
  Sorry to unload all these ideas on you. You don't have to read a lot or know a bunch of stuff to do this practice. But I think it's useful to have some rough idea of what to do and what not to do. Once you get the message, hang up the phone... And raise the great doubt!

Unfortunately, I'm not experienced in this practice. I typically stick with nianfo and mindfulness of breathing.

Master Sheng Yen did "write" a book on it. This is the book.. It's a collection of talks from hua-tou retreats, and, if it's anything like his Silent Illumination book, it'll lead you in the direction you need/want to go.

If you have been meditating for a while now go read Master Shengyan’s “Shattering Great Doubt”. It’s the most accessible book on meditating on the Keyword or a Huatou, in this case it’s “what is Mu?”.

https://www.amazon.com/Shattering-Great-Doubt-Practice-Huatou/dp/159030621X