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!
Searching through the book files I was not able to find any `\href` commands whose displayed text is a URL other than these two, so I don't think there is much to change.  Since we had one other vote for fixed-width and no dissents, let's change these two to something like `\href{http://...}{\url{http://...}}`.