I don't know of any on the horizon, but the last batch of books that were updated for Rails 2.0+ were released earlier this year. So it may be a while before any books cover 2.3.
Your best bet may be starting with Agile Web Development With Rails and then filling in the blanks with the release notes. This book was published a month before 2.3 was released and covers Rails 2.2. there are a lot of changes between 2.2 and 2.3, but nothing that is both game changing and wasn't already available as a plugin.
Most of these changes were either performance improvements or methods recognizing new options. The biggest of all these changes was nested attributes/objects. There's plenty of documentation about it, most importantly, a github repository that gives you a working example to play around with.
The short version is: Agile Web Development With Rails may not cover Rails 2.3, but it's close enough.