In 5e, can a Pact of the Tome warlock with the Book of Ancient Secrets invocation inscribe known ritual spells into their Book of Shadows?
This is useful if permitted, as it allows Tome warlocks to move prepared spells (which happen to be rituals) to their Book of Shadows, and to replace the spell with another warlock spell. Although no longer available except via ritual, the warlock now has a wider variety of spells available throughout the day.
This seem to hinge on the interpretation of "when you find" (emphasis mine) in the following
On your adventures, you can add other ritual spells to your Book of Shadows. When you find such a spell, you can add it to the book if the spell’s level is equal to or less than half your warlock level (rounded up) and if you can spare the time to transcribe the spell.
Are already-known spells included?