My recommendation is to work from the book however both are possible.
Book in InDesign is the collection of the documents so , you can work on the document outside the book or from within the book.
When you make changes to the documents outside the book then you will see the Yellow exclamation mark in the book panel infront of the Document name, on hovering your mouse pointer on Yellow exclamation , it will show you the reason as the document has been modified outside the book.
so, it is better to work from the book to avoid such issues which can be easily missed but there will be no big challenge or issue you will face if you work cautiously.
Thanks Mannish for the quick reply.
What you suggested is what I've always done before, but the last time i did this on a book file of 139 pages, made with 21 document files, the changes didn't register and the next time I opened the book or document files, I was back at square one.
The result was that I deleted the book file and made the changes in the document files and created a new book file from the revised document files.
However I've noticed an error in the new book and am wondering should I correct it in the book file or the document files?
Despite your earlier problems, a "book file" is nothing more than a list of the separate InDesign documents that make it up. You cannot 'make changes' in a "book file" (other than the ones available, i.e., add, remove, or change documents).
If you edit one of the book's documents while the book file is not open, it will still register that next time you open it -- in that case, the edited file will show a yellow warning triangle. All you need to do to "fix" that is open the document (and I say "fix", but actually you cannot do anything else without opening the document in question -- in particular, when you do anything that requires the document to be opened anyway, such as printing or exporting, the edited document will be opened and used).