At level 3, Warlocks with the Book of Ancient Secrets eldritch invocation would be able to start collecting ritual spells from all the other magic classes:
Choose two 1st-level spells that have the ritual tag from any class’s spell list (the two needn’t be from the same list). The spells appear in the book and don’t count against the number of spells you know. With your Book of Shadows in hand, you can cast the chosen spells as rituals. You can’t cast the spells except as rituals, unless you’ve learned them by some other means. You can also cast a warlock spell you know as a ritual if it has the ritual tag.
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.
Since a Warlock has to be level 3 and have the Pact of the Tome, am I correct in my understanding that they are unable to learn any further level 1 rituals, other than the original two spells picked when first acquiring the invocation?
To be clear, my confusion is regarding the bold text in the invocation above. If you can only learn new rituals of your warlock level or less than half rounded up, then at level 3, I would only be able to learn level 2 and level 3 rituals spells. As half of three is 1.5, rounded up = 2.
So, if a warlock can only access the Book of Ancient Secrets at level 3, and you can only learn rituals at your level or less than half rounded up, are you limited to just the two first level ritual spells upon taking the invocation of Book of Ancient Secrets.