Would a spell to destroying writing be perdo herbam / perdo terram or perdo animal. This spell will be used in a book that players are making
If it's to destroy the writing, and not the paper, then it should be PeTe, or PeHe, or PeAn, depending on the ink used. But I feel like just erasing the writing should use target Part.
The problem is that "writing" is not intrinsically recognisable by Hermetic Magic. It belongs to no Form.
You would need a specific spell that specifically targetted the "ink", but avoided the substance it is written on.
Different spells for
a) oak gall ink on vellum,
b) squid ink on rice paper,
c) tracings in condensation on a mirror
d) rock carvings
e) an SOS in driftwood on a beach
All the above spells would target also spilled ink, scribbles, and artwork not conataining writing.
The players are working and making a set of two books as magical items. Based on the old spell called Twinning the Tomes Level 35 Muto Animal Req: Intellego, Imagonem Ritual. was a way to communicate better between Covents. So they would only need it based on the two books that are being made.
What about using two enchanted wax tablets holding or being ACs to each other, and replicating changes in the wax to each other? That is ReAn or MuAn, does not need expensive rituals and should be simpler magic.
It still has the flaw of having ACs between covenants that can be used for other things than just communication.
p 97 Covenant: Creating an Empty book from a Filled One, PeAn3.
It does exactly that: shave the thin upper layer of a book, removing all inks, turning the book into a palimpseste. Finesse roll is required to do a proper job.
So if the support is not parchment, the Form would need to be changed, thus similar approach can be taken for clay, wax, stone, wood, papyrus. Each time the base level would be slightly different and you might adjudicate that if the material is quite thin, the end result is more fragile (depending on the Finesse roll for example).