This section of the book links to media.pragprog.com/titles/dswdcloj3/code/guestbook/test/clj/guestbook/test/db/core.clj and talks about test files. This is an unusual convention for structuring and naming tests. If you have a guestbook.db.core source namespace, nearly all tooling will expect the test namespace to be guestbook.db.core-test and so the location of this file should test/clj/guestbook/db/core_test.clj
Thanks for pointing this out. It will be fixed in the next release.