I've hitherto only run CoC as one shots - fiasco scenarios where it was expected everyone would end up insane or dead. I'm considering running a CoC campaign for the first time, and it's causing me to question a bunch of rules that I never had reason to question before because they didn't really have impact during a typical haunted house type scenario.
Previously, I never cared particularly what a mythos tome actually did because they typically showed up destroyed someone's mind and that was about it. If someone managed to get 12% Mythos Lore studying a book, or if the book contained a fact relevant to the plot neither was a problem.
But if characters are going to go from scenario to scenario, this presents for me both a gameplay problem and a conceptual problem.
From a gameplay perspective, it would only take reading 5-6 mythos tomes to level yourself up to where you could produce almost any fact - and at the same time level yourself out of the game, as your fragile sanity would not be able to take it. Indeed, by the RAW, the Necronomicon alone has a reference fact to 80% of the questions that might come up, which is paralleled in general applicability by only the Junior Woodchuck's Guidebook. This hardly seems to be enough manuscripts to make up a campaign. While I'm totally OK with the mechanics of sanity loss and decreasing stability, the speed that you figure out all the secrets of the universe seems to be not only a bit bizarre but down right counter-productive to a good game. One stint in the British Library, and you come out an emotional wreck but able to identify what Cthulhu had for breakfast the last time the stars are right and who Hastur dated in high school.
From a conceptual perspective, this is even more bizarre. I would never design a system such that if an investigator read the Oxford's History of the American People and four or five other thick books, they'd come away with 80% in history. We'd expect an actual historian to study scores and scores of books to get in the region of 80%. And this is true of any other subject as well. Yet, conceptually, Mythos Lore ought to cover more knowledge than exists in all the other subject matter combined. Only a cursory understanding of the vast range of human knowledge can be gathered by reading through the Encyclopedia Britannia (if anyone can remember what those tomes lined up beside each other look like). Better can be achieved by reading every page of Wikipedia and following every link to every source. But even that is a drop in the bucket compared to the galaxy spanning multi-dimensional multi-species lore that 'Mythos Lore' encompasses. All of human knowledge is to mythos lore as the first volume of an encyclopedia is to the entire library. The greatest of human sciences are but rudimentary primers. So what gives reading one Arab madman's graduate student thesis paper and coming away with a working knowledge of so much that vast alien lore?
I'm thinking that each time you read a mythos book, the sanity loss is per the RAW, but the increased mythos lore is reduced by a fraction that depends on how much you already know. So for example, after reading the Liber Ivonis you might gain +13 lore, but upon adding the Cultes des Goules to your reading list your lore increases by but half the usual number as much of what you are reading is no longer quite as revolutionary. So instead of adding the full +14 for a total of +27, you had +7 for a total of +20. Then, if you're made of stern stuff and read De Vermiis Mysteriis as well, but a third of the usual understanding (+4) is added bringing your total Mythos Lore to +24 (rather than +39). By that time, most investigators will probably have killed themselves casting an ill-advised spell, or sent themselves to the insane asylum, but most of the mysteries of the cosmos are still mysteries. The more books so read the bigger the differences we'd see, as each book eventually adds but 1 or 2 points of understanding - the same sort of advantage a player might gain by studying a more mundane book.
Obviously, some tinkering in actual play may be required to make the order your read things in not matter too much, but rather than outlining an algorithm I'll just deal with that in a case by case basis.
What do you think the outcome of this change will be? For example, will this make spells too accessible, and is that even a problem?