So, ritual spells don't have formulae listed...

Does this mean you can only learn them from an Instructor?

I think the spell is the normal spell. If you learn a spell (keyword) ritual, then you can ritually spellcast any spell that you know from that category with it (as long as it fits within the ritual description. You would also be able to cast the known spell normally.

So you can normally learn the spell, for example for Curse you could learn Agony with Formula or an Instructor, but how do you learn the Ritual spell Curse? If you are claiming that Curse is a Illusion category and then illusion is the formula price you may pay I can see that. But what about Ward or Watcher for example?

It's not a different spell. Just a different approach to casting it, for a different situation.

Learn the spell once, cast it both ways.

You still have to learn Curse to go along with it. You cannot just learn Agony and be able to automatically use the Curse Ritual. The main thing with the line that deals with this is that they're distinguishing it from preparations, which take an entirely different version of a spell. In order to cast Agony every way possible, you would have to learn Agony (Spell), Agony (Preparation), & Curse.

@DireRadient: Does seem an oversight. Although since rituals are usually group activities, maybe they are supposed to be learned through an instructor.

Under the Learning Spells section you'll see that a ritual can be learned from a formula or a teacher.

It's a good catch that ritual formulae aren't listed on the two Magical Goods tables. Ritual Design is also not listed under the Arcana skill, even though it reads that formulae for "all manner of magical manipulations" can be created with it. I suspect that the ritual rules were worked out after those parts of the book were written.