Is it appropriate to suggest to a professor how to communicate more clearly? I've recently began working as a research assistant (in the humanities), mainly helping a professor with a book she is editing and with lecture notes for a future course.

The problem is that it is sometimes very unclear to me what she wants me to do. For example, when communicating by mail (which we do often), instead of saying "Please do X" (which I realize only after asking for further clarifications), she might say "We will do X" or "There is some importance to doing X"...

It is quite obvious that she has very little experience in this kind of work relations. I, on the other hand, have some experience in managing people (i.e. in telling people what to do), and I think she could use a few pointers.

The question is, is it appropriate (and if so, how) that I give her some specific suggestions (such as the above) in order to communicate with me more clearly?

I'm on good terms with her and she will possibly be my advisor quite soon, and I don't want to ruin this.
For now, I would suggest with responding to statements such as "We will do X" or "It is important to do X" with something along the lines of "Agreed. Shall I start working on that now?" Perhaps she'll take the hint and start communicating more clearly, in which case the problem is solved. In any case, over time, you'll probably get better at understanding which of these statements are general statements about the future and which are implicit requests for action.

If asking for more clarity doesn't work, have a chat with her and explain that you find it hard to tell when she's asking you to do something and when she isn't and asking her to be a bit clearer. I'm sure she won't be offended and, if she is, that's a huge sign that you need to find another advisor!