Do you need to know about Mambo specifically or Joomla? I haven't played with either in a while myself, but the last time I looked Joomla had a large, active community and there's some good documentation online. Personally I always learned the most by installing it on my server (pretty easy since many hosts have a 1-click install feature for it) and diving right in to play around and see what it can do.
For me I found it had a lot of things I didn't need, and there was no easy way to integrate other scripts with joomla to the level of integration I needed. I believe it to be a solid script, with a good community. It just didn't suit my needs personally.