Jeanne Krochalis, an associate professor of English at Penn State, wrote an interesting paper on the Codex Calixtinus. I copy some of it here.
"There are lots of cheap surviving medieval guides to Rome," Krochalis notes.
But when she and her colleagues studied the 12 manuscripts that remain, they learned this wasn’t the case with the Codex Calixtinus.
"Like a Baedecker’s that never was published, the Pilgrim’s Guide did not see the wide distribution its sponsors apparently intended, and that scholars had assumed it received. The survivors date from the 12thC (three copies), 14thC, late-15thC, early 16thC, 17thC, and 18thC. Each was hand-copied from an earlier manuscript, in a tedious process guaranteed to produce errors.
Most of the manuscripts were copied directly from the original at Santiago, which means that, instead of being the forerunner to Frommer’s, the Pilgrim’s Guide was a curiosity read only by the clerics, historians, and antiquarians who had access to the monastery library. Only one copy of the Pilgrim’s Guide was made during Gelmírez’s lifetime; it was sent, along with a bone from St. James’s jaw, to Bishop Atto in the North Italian city of Pistoia."