The 9th and 10th books are designed to use interchangeably, they are divided by themes (I guess all of the grades are that way:) so you can pick and choose what you want to do. They don't necessarily build sequentially.
And we tend to do all of them orally as well, except for the writing exercises. Generally we do it weekly (sometimes less frequently:rolleyes: and she reads several chapters at a time, then we do alternate exercises every day. It's not time consuming at all, in fact that may be to our detriment because it tends to fall through the cracks :)