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?
Best Answer
No.
You can inscribe spells that you find. Which implies they must be written down. As a warlock spells you know, you gain magically through your pact to your patron. Those you don't understand in a way that can be expressed in text.
This is also been answered by Jeremy Crawford here: http://www.sageadvice.eu/2016/04/26/book-of-shadows/
Work Around
If your DM allows scroll scribing/magic item crafting, you could create a scroll and then copy it into you Book of Secrets. This could be very expensive as Common items (Cantrips and level 1 spells) require 100gp, and minimum level of 3, all the way up to Legendary items (9th level spells) requiring 500,000gp and a minimum level of 17. 5th level spells (the max for a warlocks spell slots) are rare, require the character to be level 6 and spend 5,000gp.