I ran across this and thought it would be of interest to others.
Most book publishing contracts can be divided into two types: fixed-term, where the grant of rights extends for a defined period of time, such as five years; and life-of-copyright, where the grant of rights extends for the full duration of copyright. (If you're unclear on the difference between rights and copyright, I've explained this in an earlier blog post.)
Complete article in the link:
http://accrispin.blogspot.com/2012/04/importance-of-reversion-clauses-in-book.html