Fixed several tiny (non-mathematical) typos

This pull request corrects several tiny typos. They are all non-mathematical, so probably do not need an entry in the errata list.

I added a few non-breaking spaces, only in places where it looked really necessary to do so in my particular printing (A4).

Merry Christmas!
This one also looks fine to me.  I suppose it might be clearer with a "that", i.e. "we trust that our readers know", but "that" is often omitted in sentences like this.