When a book's chapters are organized into several "parts" that each relate to a different problem, topic, or stage of the journey, it's common for a reader to read the introductory chapters, get oriented, and then skip ahead to whichever part seems most relevant to their current goals.
This means that each of the book's Parts should be designed as if it was a smaller book in and of itself, complete with its own small table of contents and introduction.
At the same time, all patterns that apply to front-loading value within the broader book should also be applied to the beginning of each of its Parts, since a reader's experience might effectively begin at any of them.
Knapp and Zeransky's Make Time, showing a mini-ToC and intro at the start of a Part to set expectations and allow navigation within it.
How it helps
When a reader jumps around, wherever they land is as important as the book's actual beginning.
If those pages leave them uncertain about where they're at, what they're getting, and whether it will work for them, they're out of there.
By treating each Part as if it might be the start of a reader's journey — i.e., front-loading the value; establishing empathy and believability; orienting and signposting — you solve these problems.
Make Time again, showing the introduction to a new Part, which reorients by anchoring to the master framework and provides context to non-linear readers via a short introduction.
How to make it work
At the start of each part, include a mini-ToC of that part, and a brief introduction.
Over-invest in the front-loading and salience of value at the start of each Part.
Use navigational aids to increase the chances that readers land at the start of a part, and not at a random spot within it, such as bleed navigation and inline indexing primarily to the start of the Parts (and/or within the current part, where context has already been established).
At each Part's start or end, make it easy for readers to figure out which Part to visit next, such as by using a map of progress.
During Beta Reading, you might also consider sending groups of readers to the start of each Part (as opposed to sending everyone to the start of the book).