I am playing a 3/3 Divine Soul sorcerer/Hexblade warlock, and have just chosen the Pact of the Tome, along with the Book of Ancient Secrets.
I have found a spellbook with Identify, which I am now transcribing into the book. I also have Unseen Servant as a known spell. I create a scroll, using the Arcana skill. And then use the scroll to transcribe the Unseen Servant spell to the Book. The Scroll is destroyed in the process. So far, so good.
I then replace Unseen Servant with Charm Person on my list of known spells.
Can I still cast the Unseen Servant as a ritual from the book?
Can I cast the Unseen Servant as a spell, using the book and a spell slot? This question arises from the interpretation from the text "You can't cast the spells except as rituals, unless you've learned them by some other means.
If this is possible, it basically means I have an external memory extension, that when held, allows me access to cast any of the entries as a ritual.
And cast the spells that previously were known spells from my spell list that has been transcribed into the book, as spells, using spell slots.
Best Answer
Any spell in your Book of Ancient Secrets can be cast as a ritual. A spell in your Book of Ancient Secrets can only be cast using a spell slot if you also currently know the spell
The Book of Ancient Secrets states:
What this means is that a spell contained within your book can be cast only as a ritual unless you already know the spell. Imagine this clause were not here, that would mean that adding a spell into your book would remove the ability to cast it normally. The clause is here is to prevent such a thing from occurring.
You can add unseen servant into your book because the feature states:
This is also gone over in the following questions:
Thus we've established that you can add unseen servant into your book and can cast it as either a ritual or using a spell slot, because you also currently know the spell. If you were to remove unseen servant from your spells known list, the clause allowing you to cast it normally would no longer apply; replacing unseen servant with another spell means you are no longer able to cast it using a spell slot.
One could argue that after unlearning the spell you still knew it and so it is a spell "you have learned" but I believe this goes directly against the intent of the feature. The book is not meant to effectively add spells to your "spells known" in this way; it allows you to add the ritual-casting version of a spell, not a spell-slot-consuming version.
I believe that "unless you've learned them by some other means" does not apply to spells that you knew at any point in your life and no longer know; it applies only to spells that you currently know.