Where can I find information on greater spirits? I must have missed it in the books, except I remember seeing some information on them with the metamagic options you get from initiating. Where can I read about the spirits themselves? I've already heard greater plant spirits are good because of regeneration. Are any of the others particularly good?

If you mean Great Form spirits, they're on page 57 of Street Magic, a product of the Invoking metamagic.

EDIT: It should be noted that Guardian and Task great form spirits are interesting, due to things you can do with their Endowment power, like having them Endow you with Materialization/Possession (and then Long Term Bind it).

Ah, yes. Thats what I meant. I'll go look that up now!