What exactly are the sources from which you can copy spells into a Book of Shadows?

Question

The Book of Ancient Secrets invocation says (PHB p. 110):

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. For each level of the spell, the transcription process takes 2 hours and costs 50 gp for the rare inks needed to inscribe it.

The top answer to this question suggests that you can copy any spells that have the (ritual) tag that you find in a wizard's spellbook or in the ritual book of someone with the Ritual Caster feat. This other question suggests that you can also copy from a scroll, including non-wizard spell scrolls with the (ritual) tag. Are there any other sources?

I'm looking for either a rules/Sage Advice/similar citation on how this works, or failing that, personal experience regarding the effect of allowing or disallowing certain sources. (Also I realize this is technically a list question, but there should be a single correct answer that is short, well-constrained, and static barring a major rules change.)

Answer 1

Looks like any written source (via Twitter):

Matt Harrah: "...For Book of Shadows warlock: what constitutes "finding" a ritual spell? A book/scroll? Observation of cast?"

Jeremy Crawford: "...Finding one written down."

A wizard's spell book, another warlocks Book of Shadows, or a scroll all qualify. I could see a DM making one carved on a wall of dungeon, etc, and it would count RAW and RAI.

Answer 2

TL;DR

Allowed sources are

• Written sources found in the course of adventuring
• Spellbooks/Book of Shadows/ritual books of wizards/tomelocks/ritual casters in your party.

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"Finding" means finding a written copy of a spell. (Tweet from JeremyECrawford)

frizbog to mikemearls, JeremyECrawford: For Book of Shadows warlock: what constitutes "finding" a ritual spell? A book/scroll? Observation of cast?

JeremyECrawford to frizbog, mikemearls: Finding one written down.

For example, a tomelock cannot inscribe a prepared spell. (Tweet by Mike Mearls, knowledgeable expert, but not RAW.)

SorcererQzot to JeremyECrawford, frizbog, mikemearls: Can they write down a spell they have prepared already?

mikemearls to SorcererQzot, JeremyECrawford, frizbog: no - requires creation of a scroll or spellbook

One written source is other players' spellbooks. (Tweet from JeremyECrawford)

LordHighMarshal to JeremyECrawford, frizbog, mikemearls How would one get a ritual spell if it is in another party member's spell book?

JeremyECrawford to LordHighMarshal, frizbog, mikemearls The warlock could copy the ritual spell from that spellbook.

Can rituals be copied from another warlock's Book of Shadows or from a ritual book? I've seen no ruling which explicitly says these are permitted. However, since "finding" means "finding a written source" and since using another player's wizard spellbook is specifically permitted, copying from another Book of Shadows or ritual book fits the same definition of "finding" as copying from a wizard's spellbook.

Answer 3

Rules as Intended, if consistency is to be maintained between Ritual Caster and Book of Ancient Secrets:

Ritual Caster

Choose one of the following classes: bard, cleric, druid, sorcerer, warlock, or wizard. You must choose your spells from that class’s spell list, and the spells you choose must have the ritual tag.

If you come across a spell in written form, such as a magical spell scroll or a wizard’s spellbook, you might be able to add it to your ritual book. The spell must be on the spell list for the class you chose, the spell’s level can be no higher than half your level (rounded up), and it must have the ritual tag.

Book of Ancient Secrets

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. For each level of the spell, the transcription process takes 2 hours and costs 50 gp for the rare inks needed to inscribe it.

The constraints are that the spell be physically written down, or inscribed, equal or less than half your level, and you have the time and materials to transcribe it. This follows the rules interpretation provided by Jeremy Crawford and Mike Mearls, where the explained the Pact of the Tome Warlock with the Book of Ancient Secrets must find the ritual spells written down.

You have already identified the prime sources: other spell books and scrolls, if the spell has the ritual tag.

Additional locations could be allowed, or rather included, by the DM for strategic reasons. A story driver ritual spell could be etched into the wall of a dungeon, serving as a key for a future dungeon.

Answer 4

Per the PHB page 110,

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 you can spare the time to transcribe the spell.

The only written spell sources are scrolls, a Wizard's spell book, and a Ritual Caster's book. It's slightly ambiguous, but a very lenient DM may accept a non-written form of a spell under 'find'. Such a thing might be a ritual spell available from a wand or staff. I would not.

The only other clearly acceptable alternative to a written spell is a spell the Warlock himself already knows.