Ruby on Rails 2.1: What's New? (PDF, 2MB) is a free e-book written by Carlos Brando (with help from Marcos Tapajos and Daniel Lopes). It was initially written in Brazilian Portuguese (find the original version here), but this version is an English translation by several community members.

As far as community written books goes, it's pretty good! It comes in at 124 pages, and while 30 of these are just the Rails 2.1 CHANGELOGs (formatted in a nice way, none the less), the rest definitely does a good job of presenting some of the new features of Rails 2.1 (such as named scopes, dirty objects, numerous ActiveRecord changes and time related functions) in a very easy to digest manner.