@Toastrz included several secrets in the Cursed Library, including some that, as of October 2019, were still unrevealed:
This was his response when I asked about them a few days ago:
What secrets could have survived over a year after the dungeon’s release? At the least, we know what they aren’t, because the following secrets were already public knowledge:
Known secrets
The Realm Eye
Shortly after release, Toastrz commented that the Realm Eye had 253 unique prompts. It took a couple of months, but those were all finally discovered.
More dialogue has been added to the Realm Eye in the year since its release, including a clue to the Inner Workings. But unless one of the secrets Toastrz refers to was added after the dungeon’s release, we can rule out a prompt being one (though they might involve the Realm Eye in some other way).
Honey Scepter
The honey scepter room was found early in public testing. It’s an Easter Egg referencing Toastrz’ favorite item, which became a meme in UGC/closed testing.
Rare dialogue
The Lips of the Lost rarely say “Tertiary”, which is a reference to an episode of Homestar Runner featuring a curse on the word. After they say the line, they appear to violently self-destruct.
Avalon also has a rare line of dialogue, “Grant us eyes… grant us eyes…”, referencing dialogue spoken by Bloodborne boss Micolash. You do not need to be near Avalon to trigger this line; it seems to be random, like “Tertiary”.
The known Easter eggs might just be that, or they might have greater significance. No way to know right now.
There doesn’t seem to be any obvious clues in the XML and sprite sheets, but here they are anyway:
If you’re hunting for crossbows, be on the lookout, but that three secrets haven’t been noted after so much time suggests that they’re devilishly well hidden (and at least one is significant enough that Toastrz knows for sure it’s undiscovered). Finding them will probably require a lot of luck and creativity. The existence of secrets doesn’t seem to be common knowledge, so link to this thread and spread the word! The more people searching, the better the odds of finding one.