The usual trick: a secret to get while sliding... Sliding the tunnell leading to the cavern with 2 lion's mouths, jump and grab a ladder.
Caution!!! Invisible wall bug! To get the secret, save before sliding, and repeat the jump-while-sliding until you grab the ladder. If Lara slides down in the cavern without grabbing the ladder, an invisible wall "protects" the ladder: you can return into the library and you can repeat the sliding, but then Lara will bounce on the ladder without grabbing it.