"Is that textbook thick?"
Don't know if this has been said anywhere on this course yet but round objects such as trees, a person's leg, or a pen all use ふとい to describe thickness. More flat objects such as textbooks or clothing use あつい. On the flip side, ほそい is used for round objects (eg. branches, pencils, legs) to describe their thiness/narrowness and うすい is used for flat objects (eg. textbooks, clothes, meat). Hopefully this helps, please let me know if I've made a mistake.
It's easier if you're from somewhere with a kanji background (e.g. China, Taiwan, Hong Kong, Malaysia, etc) : 太い、厚い、細い、薄い
Since I can't favourite this comment I'm taking a screenshot for now, thanks!