Some of the manuscripts I have were for spells the outsider already has, so I just sold them, but now I have one to allow me to speak with the dead. This sounds useful, but I don't know what I'm supposed to do with it in order to "research" it. Alternatively, maybe I should just sell it, since i'm desperate for cash if it's not critical to accomplish a quest.

If your occult skill is high enough, you can research it during camping. Once complete you'll learn the spell.

In your case it sounds like you only use the outsider for spells, so make sure his occult skill is high enough and get him to research it.

However, I believe this particular spell may actually require the PC to learn it in order to use it - I'm pretty sure you can't get one of your followers to use it, at least that's what I remember. Which is pretty crap IMO.

It's definitely not essential to anything, though I do recall it opens alternate paths through some quests or something. Definitely nothing I missed when I played through the second time without it.