Documentation is included with each release. That is currently all the "standard" documentation currently available. However, EBE team members blog periodically about different aspects of the project and you can access the specific blog postings using the links below.

CKS:EBE 2.0 Beta 1 Posts:
Lawrence's announcement on SharePoint team blog
Vince's post
Jag's post
Ari's post with pictures of custom themes
Brent's post with screencast overview

OOTB Blog Template Customization Posts (not CKS:EBE):
Nadeem's initial work on trackbacks (to be included in Beta2)
Ari's custom blog master page
Ari's custom RSS feed
Mark's series on blog customization