When you read the chapter titles, you will think, "no problem." Believe me, this group of people are great at writing solid, challenging texts that are fantastic for math enthusiasts.
ETA: just realized that you didn't want to do Algebra or Geometry.
It might work well amongst an advanced class, a tutorial group or a math club where kids have the support of each other as they struggle with the complexity of the problems, but for a young-ish math-minded homeschooler it would require a really amazing capacity for persistence and hunger for challenge.
It very quickly gets into complexity more typical of a good Algebra 2 course, so I would think it would be a good next step after Key To Algebra is completed if your dd feels she's ready for more algebra challenge at that point.
(I haven't tried Ao PS's geometry program, so I can't say if the same would be true of that, i.e.