I play a 3/3 Divine Soul Magician / Hexblade Warlock and have just selected the pact of the volume along with the Book of Ancient Secrets.
With Identify, I found a magic book that I am now transcribing into the book. I also have Invisible Servant as a known spell. I am creating a scroll with the Arcana skill. Then use the scroll to add the Invisible Servant spell to the book. The scroll is destroyed. So far, so good.
I then replace the invisible servant with the wizard on my list of known spells.
Can I still cast the invisible servant as a ritual from the book?
Can I act as a spell on the invisible servant with the book and a magic slot? This question arises from the interpretation from the text "You can only use the spells as rituals, unless you have learned it differently.
Basically, if this is possible, it means that I have an external memory expansion that, if held, gives me access to perform one of the entries as a ritual.
And Cast spells, previously known as spells from my spell list, that were transferred to the book as spells using spell slots.