Although The Pyramid Principle by Barbara Minto is always recommended as a must read on structuring, I personally couldn't get through the first pages as it seemed so heavy and boring to me.
Besides, any reading is passive learning which is the least effective method to acquire a skill ever.
Instead, I recommend active learning such as practicing structuring drills, doing cases with peers, taking sessions with a coach, e.g. everything that forces you to actively practice instead of passively consuming content.
Also, try applying structuring principles in your everyday life, for example, try structuring:
- all the kitchen equipment and utensils;
- all the clothes items in the wardrobe;
- all types of groceries in the supermarket etc.
Hope this helps,