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.)
Best Answer
Looks like any written source (via Twitter):
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.