The Book of the Popes or the Liber Pontificalis is a major source for early medieval history and one that has received intense critical scrutiny. The simplest view of the book is as a series of brief biographical entries on the popes up to the late 9th century arranged in chronological order, each consisting of the number of years of service (from which the regnal dates can be deduced), place of birth, parentage, the corresponding emperors, building campaigns (especially of Roman churches), ordinations, major pronouncements, place of burial, and the time of vacancy before the next elected pope was consecrated. However, the process of composition precludes naive reliance on the book for historical information.
Because the Liber Pontificalis was produced by minor officials of the papal court the evidence has been sifted for signs of bias, suppression, and falsification, all of which are found in quantity without invalidating the material as an historical source. The entries for the first three centuries are probably most useful to historians as examples of what was known in the 5th century about the early church. From the 4th century forward the compilers are on more secure ground, though there are still obvious discrepancies and mistakes. Textual examination suggests that there were two early versions before the siege of Rome in 546, after which the Liber Pontificalis was untouched. From the early 7th century (roughly the time of the pontificate of Honorius I) forward the entries are contemporary, added shortly after the death of each pope, and although reflecting biases of the authors are at least reasonably accurate.
The work of many compilers and authors over a long period complicated the process of creating usable scholarly editions. Louis Duchesne and Theodor Mommsen each produced editions (Mommsenís is incomplete) at the end of the 19th century. Translations and further commentaries appeared throughout the 20th century devoted to discovering the levels of historicity in the entries.