Default WordPress theme customization

Before WP 3.x, I used to take a copy of the default theme and customize it as our site theme...also WP help offered detailed information on each and every file inside the default theme

Since WP 3.x, default theme is gone and now they have 'twentyten' theme, which contains a lot of extra files and no help about those extra files in WP

So can someone please tell me:
1) Where can I find more information / help to understand each file in the new default 'twentyten' theme?

2) Is there any online documentation that explains how to customize the twentyten theme?

Thanks in advance for your time and efforts