Advice on subclass feature

I'm making a rogue subclass. The flavor is an occultist that uses ritual magic. One of the 3rd level feature is just a copy of the invocation book of ancient secrets.

I´m thinking, should I nerf it by limiting the number of ritual spells the subclass can acquire?

How is it different from a Phantom Rogue with the Ritual Caster feat?

Not being related to dead people, and presumably having more rituals.

Ritual caster can only get spells from one class.

Ah, so it is meant to be more of a Book of Ancient Secrets. As long as they only get a few rituals for free and have to go find the rest in the world to copy then I think it's perfectly fine - the power is then controlled by the DM and how many scrolls / spells they want to make available. Though you will need to put in some rule for the level of the spell, because ritual spells being "learnable" generally follow full spellcaster scaling, so they could only copy ritual spells equal of a spell level equal to half their level or lower. 

I would not give them a feature that allows them to automatically know all ritual spells, or allows them to copy/learn ritual spells of a higher level than normal level scaling. 

I hope there are some other cool subclass features though, because ritual spells aren't all that exciting in 5e, and it's very likely other members of the party will have access to the same rituals.