There are several different types of layers in my writing world. I think sometimes, I may confuse people when I use the word. It needs to be seen in context.
First of all there are plot layers. These are different plot lines for the main characters to enact that are different then the main plot line. they are not complications to the main plot, but can impact on the main plot. They are also not subplots. Subplots are enacted by secondary characters, but plot layers are different problems for the main character to solve. It means that more than one thing is happening to the main character.
So in A Christmas Wedding Wager, Emma has several different plot layers. Some of these are: 1. She is attempting to get the bridge built and struggling against a number of forces.
2. She is forced to confront her place in society and decide where she wants to be. 3. She also needs to protect her father and to ensure the company survives. 4. the main spine of the story -- her growing relationship with Jack. It should be note that Jack has several layers as well.
The main spine has also layers and complications. In other words, it is never straight forward. In ACWW for example, the relationship is coloured by events from seven years. Events that each has a different memory of. With complications, you need to think how can I make this worse for the protagonist. And ultimately, how can I make this relationship fail...
Secondary characters may have several different layers, but they will have fewer layers than the main characters or basically they will take over the story. As an aside it should be noted that secondary character are often seen through the filter of a protagonist's point of view so they may appear flatter than they actually are. Every character is the hero of his or her own story.It just depends on which story the writer is telling.
The way I bring the plot layers and subplots into the main spine is called the nodes of conjunction. In other words how is the thing woven together. This is when scenes/places/characters are made to do double or triple duty. Story is all about conjunction and connection and not coincidence. Coincidence may play a part, but ultimately the story needs to driven by action/reaction. Donald Maass in his Writing the Break Out Novel Workbook has several useful exercises for helping with this.
Then there are the writing layers and subtexts, the subtle shades of meaning. As McKee says -- there is a maxim in the theatre if this scene is about what it seems to be about, then it is in trouble. There needs to be more. There needs to be depth.
I write fast. I tend to get down those things that I need to get the story written. Then I go back and add what is needed to up the tension. Story is one place where logic can work backwards. You reach the end, and suddenly you think -- ah that is why this or that was important. You then go back and make sure the foreshadowing is there. You can add (or indeed take away) emotion and meaning. In other words, unlike life, the writer can go back and change the past.
So hopefully you can see that there are layers and then there are layers.
I would argue strenuously that category novels are just as likely to have layers, and that it is all in the execution. Because of the length of a series novel, and the need for focus on the main spine, it may have fewer subplots. But again it depends on the skill of the writer.