Can somebody suggest a good tutorial on Git? I've found many simple guides with a plain web search, but I'm already very familiar with other version control systems and I'm looking for documentation on the internals.
From the sounds of it, this should be exactly what you're looking for: Understanding Git Conceptually.
A good introduction to the concepts is Git for Computer Scientists.
I also found Git from the bottom up (PDF) very useful when I was first learning Git. That paper explains exactly how the Git internals work, including blobs, trees, the index, stashing, the reflog, etc.
I've found git's documentation page extremely useful, it includes talks by Linus.
If you google "git tutorial", the official tutorial is the first result. It's excellent.
I keep saying this, but I was (and still am) an absolute beginner when it comes to version control, yet I got up to speed with git and I can use it "effectively" (at least I think I do). It's practically my first version control tool! I've used svn a bit before but I find it very confusing, git made a lot more sense to me!
if you want to really dig git you need to understand how it works behind the scenes. try http://ftp.newartisans.com/pub/git.from.bottom.up.pdf I found it to be a good guide into inner working of git.
In addition to the free Git from the bottom up tutorial I highly recommend the Git Internals PDF (US$9).