I just installed PostBook acording your documentation. On Ubuntu 7.10 I ran into problems. When trying to connect I get:

QSqlDatabase: QPSQL7 driver not loaded
QSqlDatabase: available drivers:

I ldd'd the problem down to two missing libraries:

ldd sqldrivers/libqsqlpsql.so
        libssl.so.4 => not found
        libcrypto.so.4 => not found

I locally linked the existing libssl (openssl) and librypto libraries to the local PostBook install dir:

user@ubuntu: /tmp/xtuple-2.3.1$ ln -s /usr/lib/libcrypto.so ./libcrypto.so.4
user@ubuntu: /tmp//xtuple-2.3.1$ ln -s /usr/lib/libssl.so./ libssl.so.4

Then PostBooks works.

-- Robert