Plants are actually extremely complex and cool organisms that have almost as many interesting functions as animals. They can move, climb, and even hunt. As you progress through the plant chapters, I’m sure you’ll start to gain more appreciation for the grass in your lawn and your half-dead houseplant!
The best Campbell’s chapters for plants are 29-30 and 35-39. In 29-30, make sure you understand all the terminology and life cycles, as these are super relevant both on their own as well and to make clear material in chapters 35-39. I would advise you learn the life cycles well enough to roughly draw them out without looking at the book.
In chapters 35-39, you need to know and be able to apply pretty much everything. For example, which hormone causes leaf abscission? Hint: it’s not abscisic acid! For example again, which types of plant cells are dead at maturity? Hint: it has something to do with lignin.
While the plant chapters here are pretty comprehensive, they don’t cover everything you need to know about plants. A great resource to learn and understand plants better is Raven’s Biology of Plants. Raven’s is pretty comprehensive, so USABO, you can probably just focus on chapters 2, 19-20, and 22-30. One of the coolest parts of Raven’s is the diagrams. Most of them are either images or penciled over sketches of pictures, both of which are much more accurate than the cartoonish diagrams found in other books. Many times, USABO tests will have questions where you’ll have to identify a feature of a plant based on a cross sectional image. Raven’s is one of the best resources for this sort of problem.