The ideal level is when there are no more than three or four words your child doesn’t already know how to spell within a lesson. You can check some of the lessons at different points in your book to get a feel for how well it fits. If you want to see how the other books compare you can also download the Spelling Wisdom free sample that has lessons from all of the books.
The passages in Book Three may be too complicated structure-wise for her even if she knows how to spell the words. Are the passages in Book One less cumbersome? (I would like her to eventually be able to read language like this with ease but we need to build up to it.)