Someone mentioned to me that materials are more likely to drop from mobs at deeper levels, specifically that the Book of Dark Rituals would be more likely to drop from a BK at d21 than d19.
The wiki says that "Higher-ranked materials have a better chance of dropping on deeper levels of the Clockworks," so this is where he may be coming from.
Can anyone confirm/deny this theory?
Well, the strata of the dungeon generally correspond to star-levels of items. For example, on Stratum 3, you're expected to use 2-star weapons and you'll see a lot of 2-star materials drop. You'll also see some 3-star materials, but rarely 5-star materials. Go to Stratum 6 if you want a lot of 5-star materials. This is all very well established, and it's what the wiki text you quoted is describing.
You are trying to apply this general principle to Book of Dark Rituals, which is a material in name only. That item is so special that it could easily be governed by different rules. You want to search Google for something like "book drop rate site:forums.spiralknights.com" (without the quotation marks). There haven't been any great studies, because it's so hard to get data, but you might learn something.
Also, this is thread is not really about editing the wiki. It would be better if moved to General Discussion. Cheers.