First, "they" who read your email is a computer program. And yes, it automatically inserts ads to the right of the email occasionally. You hardly notice them. As for privacy concerns, you know that email sent on any email system is just as vulnerable to prying eyes as gmail? If a *person* wants to read your email and has the authority or know how to do it, they will. If the feds want your email, they will get it, hell if they want your text messages they can apparently get those too. If a computer program scans my email so that it can do targetted advertising, I am perfectly fine with that. There is no human who can read your emails on gmail and not read them on another server. Even when you've "deleted" your emails, they are often still there for months or years, accessible if needed.
Do you use google? You know they target ads based on everything you search right? It's no different, except that instead of your search queries they also advertise based on email content.
I wouldn't reccommend going into your public library and checking out 100 books consisting of books on chemistry, religion, atlas', psychology, terrorism, and law enforcement tactics. That's going to set off some flags. That's a privacy situation I would worry about. That is people directly reviewing your reading habits and deciding if you are a concern or not. This is a whole other beast.
