I totally second reading the book front to back. It is not that long and fairly well written, and can save so much time and frustration in the long run. Giving the link is the first thing I do when helping someone with git.

Unfortunately so many people put that aside and never read it, and then come back to me a few weeks later because they messed everything up again, and still do not understand what I mean with "index".

Git is very powerful but a bit awkward, so I consider knowing the underlying concepts necessary to use it properly.