You have several ways for doing this.
- “Duplication” of original file
Open your original file. Delete text which is not part of the chapter. Save this chapter under a new filename. Close. Restart for a new chapter.
It might be wise to duplicate the original file under a new name to protect against a personal mistake and prevent loss of original material. Then you don’t need to change the filename after deleting material, which can be safer than the previous procedure.
Open the original file. Create a new blank file for a chapter. Copy text drom the original file. Paste into new file. Save.
As long as the copied block spans the footnote anchors, footnote contents is also pasted.
== Recombination
When you have split the book into chapter files, the smart way to recombine is to list the chapter files in a master document.