The noun liber cannot have the sense of "book" in ancient Rome, so for the next iteration of DL Latin, please consider glossing librum here as "parchment" or consider volumen to approximate "book." Here's a link for people who are interested in the matter: https://penelope.uchicago.edu/~grout/encyclopaedia_romana/scroll/scrollcodex.html Cicero's library, for instance, consisted of rolled-up scrolls. "Book" in the sense of a bound volume with pages is anachronistic to the Roman classical period.
There is nothing wrong with using book for liber. A book does not always bound pages. One of our contributors had a book published digitally a few months ago. It was a book before it was ever printed. It's a book in the sense of a long work that is fit for publishing.
Using parchment would make this course less accessible to learners.
Here they give an in depth explaination of books, volumes, parchment and paper etc.
LIBER (βίβλος, βιβλίον), a book. But it must be recollected that these words in Greek and Latin until a very late period mean a book in the form of a roll, as will be explained below, and that the modern book shape was used only for the codex
They explain a lot more but I think that was the most relevant part.