Double posting is generally not well liked is many forums unless you have a good reason to do so. I'd classify this one as a good one.
There's no null in C++ and I don't think there will ever be. "null" is a string, not literally null, so that's why it won't work.
But you can just set the string to "" (empty) and then check for empty if you want. But you can just replace the data inside the string with new data.

I'm not very familiar with any newbie C++ books, but I do have about two good advanced books. Anyway, the good books sticky should help you find some good books.