With that in mind, let's take a look at palimpsests. A palimpsest is a product of (usually early) medieval economy. Vellum wasn't cheap, and sometimes monks or whoever couldn't lay their hands on what they needed to make new books. What they would do was take an old manuscript they didn't need (or couldn't read) and wash off the original text, then write over it. However, the washing process didn't work perfectly and the text would often reappear as a kind of "ghost" under the original one. This example is from a text called the Codex Nitriensis:
See how the original text is still faintly visible at right angles to the new writing?
Now, in order to make the old text visible, they used to dose it with all kind of chemicals, but today they use various spectra of light to make the ink show up more strongly, photograph it, adjust them digitally ... it's much more effective and much less destructive.
By the high middle ages, they were scrubbing that stuff off with pumice, so palimpsests from that era don't survive as well.
It appears that monks may have been particularly interested in writing over pagan or heretical texts -- these things were supposed to be destroyed but the vellum couldn't go to waste. It might even be that they felt like they were somehow sanctifying them by writing over them? Alternatively, they may have just seen those texts as unimportant or irrelevant, but for gaming purposes the "purifying" thing is more appealing.
I quite like the idea of making the detection important. Modern historians read palimpsests using clever new imaging technology. You could go all From Beyond style on it -- some scientist develops a new method which is the only one that can read this strange palimpsest.
The "Red Book of Darley" is not a palimpsest, but I couldn't resist this quote: "This booke was sumtime had in such reverence in darbieshire that it was comonlie beleved that whosoeuer should sweare vntruelie vppon this booke should run madd."
And, of course, if the tome was written in some kind of strange alien language that the monks couldn't read, they'd be more likely to scrub it off and write over it, which would explain why the remnants of evil prehuman civilisations would be found as part of a palimpsest.
Anyway, here's a sample palimpsest text for Call of Cthulhu.
The Beornwald Benedictional
Associated with the church of St Beornwald at Bampton in Oxfordshire, this benedictional (or prayer book) contains a series of prayers and blessings for different occasions, presumably to be used by a priest associated with the minster. It probably dates to around the very late 10th century or early 11th century. The text contains no Mythos-related content, but a successful Occult roll (or History at -20) will suggest that there are rather a lot of apotropaic blessings compared to other books of this type.
Overwritten by the Benedictional is a document written in flawed Latin, the Annamoris Letters. These letters contain reports of a spiritual discussion between one Annamoris, a British priest from an isolated community somewhere in northern Britain, and a relative in London who had recently converted to Christianity. The two debate the differences between their religions. Annamoris describes a number of ritual practices, including a sacrifice to certain human-like creatures which live in the nearby hills. Annamoris specifically defends these grotesque and barbaric rites on the grounds that they work, unlike the rituals of most Romans. His nephew, Cunomoltus, angrily denies it, but in the later letters it's possible to see that his confidence is somewhat shaken.
It should be possible, through dedicated historical research, to pinpoint the location of the rites Annamoris describes for contacting the degenerate serpent-beings who taught him his Mythos knowledge.
The Annamoris Letters: -1d3/1d6 SAN, +3 Mythos; x2 spells; 4 weeks
Spells: Contact Serpent Folk, Shrivelling, Voorish Sign