In my opinion, you've touched on a sore spot in GTD. The entire system is well described in the context of obvious cases where the next steps of a given project are atomic in nature, e.g. Project - Changing tires: Look for a mechanic's phone number on the Internet, Call a mechanic, etc. It's worse when it comes to using GTD in a deeper sense. In the GTD system, there is an obvious tension between flexibility and direction. If we were to approach it literally, when deciding what to do, it is first the context that decides, then time, energy and finally priority. Contexts have clearly become blurred for us. They made more sense at the beginning of the 21st century, but today they have shrunk or expanded significantly - as you prefer. On the one hand, 80% of the average person's affairs can be crammed into the computer context or even online, so some use a more detailed division within the computer context, i.e. according to the tool.
GTD is like a hammer. It is a tool (here a framework). And most often on the pages there are instructions on how to use this hammer: collect, clarify, etc. However, there is very little information on what can be built with this hammer. David Allen was probably the most specific in his first book. Now, when I listen to his statements, I have the impression that they are so general that they are sometimes on the edge of some philosophical considerations.
Honestly, e.g.
@Oogiem can be more inspiring here.
Back to your question. A lot depends on why you want to read War and Peace. If it's just for fun and without any deadline, then you can write down your next action as a bookmark, for example, as "Read War and Peace from page NN/1500". Time is irrelevant here, energy is also irrelevant, and if you always have this book with you, for example in digital form, then honestly, the context is also irrelevant. The disadvantage of this solution is the lack of any direction. Nothing guarantees you that you will ever read this book.
It remains to be hoped that you regularly review all your lists during the day and at the same time you will feel like reading.
The second option is that you want to read this book for pleasure but, for example, in 3 months you have to return it to the library. In my opinion, a flexible approach is not the best. This is where you need direction. Dividing the book into chapters seems to be a good idea because a chapter is a coherent concept. You also need to plan your reading in time, for example using timeblocking. Here, the reading time will definitely come into play, for example 30 minutes per chapter.
To sum up: the more direction we need, the more we need to use external techniques. The model of context, time, energy, priority + intuition that Allen talks about when describing the process of making decisions about what to do - is not enough to guarantee "success". In many cases, you simply have to plan your work.
PS. I wrote on this forum once before that GTD currently needs a deeper look, but not so much philosophically, but more in the spirit of: study cases. All these websites, blogs, or interviews (even those conducted with Allen by Dave Edward here) practically - in my opinion - do not add anything new. It is like going around in circles.
I recommend, for example, Cal Newport's book - "Deep Work" (I am translating the title from Polish, maybe it is a bit different in English) to see how you can convey something fresh and practical. Of course, this book does not solve everything - it deals more with the deeper content of what we do and requires supplementation with some system such as GTD, timeblocking etc.
