Books for Mac OS X 3.0 Alpha 5 has been released by Chris Karr. Books is a free book catalog system for Mac OS X (10.4+ although it may work on 10.3.9).
This is the first “wide” release of Books and features some expanded functionality in the import and export plugins, the introduction of the Tab-Delimited Text export, and various bug fixes.
Update: It looks like it is no longer in active development and/or will not run on newer versions of OS X.