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.