Apparently a number of Books of Old written by Birenni, one of the great D'ni writers, some of the Healer Ages that had been lost since the arrival of the D'ni people. It is said that there were tremendous searches made for the books but to no avail. The books were a tragic loss felt by the entire society.
During the time of Ahlsendar's Plague, King Hemelin was fairly convinced that a cure lay in the Books of Old, and he had been encouraging the Guild of Healers to look there, seeking the Book of Birenni. His nephew King Behnashiren had been hesitant to allow access to the Books. Rumors say that while Lalen was searching through the Books of Old she found hints as to where more of the Old Books had been kept. It was a few months later that the Book of Birenni was found, on Leevobro 12 of 2262. Healer Jaysem worked day and night for six years until a cure was found.
in 2807 King Needrah commissioned the design for a monument to the discovery of the Book of Birenni, which Lahkeer took to prove his legendary skill to the public. The finding of the books was commemorated by hiding fire marbles in houses and Ages for children to find.
The Books of Birenni are mentioned in an old website by Cyan. They are referred to in the plural as "a number of books". Their discovery is singled out as a significant event in the D'ni history, but only culturally.
The event is mentioned again the King Hemelin notebook, but here the Book is in the singular, one of more "Books of Old". The significance of their discovery here is only in the context of the cure of the Plague.