CMMI is still under definition (I think) but there is a good book out on it, CMMI Distilled, by Ahern, Clouse, and Turner. I got it on Amazon.com so I'm sure you can still find it there. The basic difference I see is that CMMI now allows you to take all the CMM models (SE, Software Acquisition, People, etc.) or only those you care about and measure your organization once, as a whole.
The difference can be described as follows:
- For CMM there are many available models: for software engineering, for systems engineering, etc.
- CMMI integrates (note the I) some of these models in one model. This is very useful for companies that needed to implement more than one model. When you look at the different CMM models, there were a lot of common things, so integrating them seemed an abvious choice.