Producing different documents from the same source file?

There are two ways to handle the situation you describe, as far as including or excluding chapter files. One is to set up two separate book files, each of which contain only the files you need for that book. This is MY preferred method.

The other is to have a single book file with all the files in it, and each time you produce a version of the book, you exclude the files you don't need by right clicking on the chapter file name in the book and selecting Exclude, or, if the last time you produced a version you were excluding files, right clicking on the chapter filename and deselecting Exclude. I don't like this method, since it requires me to remember every single time that I have to either remove or add the exclusion. It does have the advantage that you don't have to create separate generated files, such as TOCs, for each book, but that's not a big deal.

Variables are actually pretty simple. You create them by clicking Insert > Variables, which opens the Variables pod. The icon to the far left of the Variables pod menu bar lets you add a new variable. You give it a name, which is how it is referred to in the document, and a value, which is your NO/YES. Any place you need either the NO or the YES, you open the Variables pod, find the name, and double-click on it. (The Variables pod is one of the things I keep open all the time, btw. It will not close unless you close it.)

Remember that Settings file I talked about? That's a separate file that I create for each output from a book file that contains the user variables and the conditions that pertain to that output. In your case, I'd create two settings files. One would have that variable contain the value NO. The other would have the same variable name but assign the value YES. When you want to change the value, you click File > Import > Formats, select Variables and the name of the appropriate settings file, and then click OK. You might want to look into Rick Quatro's Import Special plugin/script, since that gives you finer control of what is and isn't imported and is pretty cheap for the value it adds.

If you see the image below, the variables with the purple gear next to them are called "system variables", and they are in every Frame document by default. The ones with the blue gear are "user variables", those are the ones you created. The Definition column shows the assigned value. The highlighted variable is one I use for the product name. This isn't likely to change for a document, mind you, but using a variable means I don't have to retype it all the time and avoids spelling errors. You might also notice that I have a variable called DS Degree Sign, which lets me insert a degree symbol without using the ASCII codes or the character palette. Since I have the variables pod open all the time, it's faster.

All variables, system and user, are editable by clicking the pencil icon on the Variables pod menu bar. Just be careful that what you edit is the Definition field! (I goof on this from time to time, and I've been using Frame for more than 2 decades now!)