I am a first year math PhD student in the US. I was initially interested in the work of a professor and I contacted him to ask for the possibility of a reading course. We didn't make any concrete arrangements because he was on sabbatical for the spring (and summer) semester and he simply asked me to follow up closer to the fall semester. In the meantime, I had a reading course with another (math) professor in a different and not closely related field and my interest shifted. I think I would prefer to spend more time exploring that field instead. Is it bad etiquette to back out now and tell the first professor that I would prefer not to proceed with the reading course with him?
This is fine etiquette, particularly letting them know as soon as you're certain just in case they were thinking of pulling together materials. If you're worried about being particularly polite, you can avoid mentioned that your interests have shifted and simply say that something else innocuous as long as it is true, e.g. that after the last semester you've reassessed your work load and don't think that a reading course fits any more.