As far as I know, "between" is for specific things while "among" is for a group of non specific things/people. For example: I am going to chose among these books. I am going to chose between the red book and the blue book. The first example, there is a group of non-specific books, you don't know specifically about each book. In the second example, you are choosing either a specific red book or a specific blue book. I know this seems confusing, but to be honest if you mix "between" and "among" up it's not a big deal and any english speaker will still understand you.

between (2 or more) among (3 or more)

between; to separate to point from each other. Among: use for involve of more than two