There is also a recent paper by Arthur Lee and Joseph Zachary called
"Adding Support for Persistence to CLOS via Its Metaobject Protocol".
There is also an old paper by Theodore Phelps, called "Persistent object
storage in collaborative systems".
In the context of pure Lisp OODBs there is (besides PLOB) an old paper
from by and Swanson called "UCL+P - Defining and Implementing Persistent
Common Lisp".