The Ritual Caster feat provides for a character to learn and use a limited number of specifically marked spells from the spell list of another class -- for instance, a Sorcerer might take Ritual Caster and choose find familiar as one of the first level spells initially in the book when acquired.
The feat also specifies that the Ritual Caster may copy other ritual spells into the spell book, from the same class list they initally chose -- but what determines the level of the spells they may so copy and use? Sensibly, it might be a spell level the Ritual Caster could otherwise cast, but that would close the feat from non-casting classes (like a plain Fighter) and reduce its utility for half- or quarter-caster classes (who advance spell levels very slowly compared to, say, Wizards and Clerics).
So, what level spell can my fourth-level Sorcerer put into the ritual spell book? First only? First and second (the level the character can cast)? Or whatever I can find (and if so, what determines if I can cast those spells)?