The book sounds good. I think ultimately there are two things that are important here: the first is teaching her about botany and the second is to instill and build on her drive to want to learn the material or more broadly a problem solving/curious mindset. In my opinion, the second one is more important.

Two pieces of advice:

• Forget about the structure. Just think about setting up an environment that will let her explore, play and teach herself. The book is a good start. Maybe, a plant for her room would be a good idea.
• Explore with her. The best thing you can do, I reckon, is to take her outside and explore with her. I don’t know much about Botany, but I think it would be cool, as an example, if you picked up a flower and pointed out to her that most people are born with two arms and then asked: “So, would that mean that the amount of petals on this type of flower will all be the same”. Then, no matter what she says you can go to a group of the flowers and let her count the amount of petals to see if they’re the same. Then, you can ask another question: are the buds the same etc.