I am trying this code:

func deleteBooks (at offsets: IndexSet) {
        for offset in offsets {
            // find this book in my query
            let book = books[offset]
            // delete it from the context
            modelContext.delete(book)
        }
    }

It works perfect with:

 .onDelete(perform: deleteBooks) 

But what if I create a button to delete a book that is inside a list? how have I got to call that funcion???

Button ("Delete") {
deleteBooks(at: ??????????????????)
}

how have I got to call that function to make it work????

Sorry if it is a really easy or silly question, but I don't now how to call it...

Do you mean adding a button of your own, not the one provided by native API, aka swipe to delete?

If so, you may create a button inside a list itself. Similar to below:

Button(action: {
  removeBook(book: book)
}, label: {
  Image(systemName: "trash")
)
.buttonStlye(.plain)

and function will be something like:

func removeBook(book: Book) {
  modelContext.delete(book)
}

It works !!! Thanks a lot!

So, this kind of function

func deleteBooks (at offsets: IndexSet) {
        for offset in offsets {
            // find this book in my query
            let book = books[offset]
            // delete it from the context
            modelContext.delete(book)
        }
    }

is needed when I want to delete a book in a list using .ondelete , isn't it?

Mark It !

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is needed when I want to delete a book in a list using .ondelete , isn't it?

That's correct!