This is a close-up view of the book in the Pope's lap.  The Latin text uses some abbreviations, but would read in full as "PER OMNIA SAECVLA SAECVLORVM" on the first page and "PAX DOMINI SIT SEMPER" on the second.  These lines are read by the priest at the end of the Lord's Prayer - i.e. during the service.

Now, as I've indicated, I suspect that the two sections of the painting that have been destroyed showed the Slavonic books which Pope Adrian II personally approved of at his meeting with Cyril and Methodius.  If this is the case, the fact that the book in the Pope's lap is open to a section meant to be read by the priest during service, rather than, say, something from the Gospels, could be a sign that he was specifically approving of the use of Slavonic for church services (as opposed to merely for translation of the bible).  More to follow.