Mercurial

Mercurials are a class of intelligent beings.

They are not bound to a physical form, albeit they are able to create one for themselves, and most of them stick to one (usually humanoid) shape unless there is good reason to change it. Many of them have command of magic/supernatural powers in addition to not being bound to the laws of physics personally.

Mercurials do not generate spirit, but have to obtain it from outside sources. The majority of them is able to absorb free spirit, but some have to steal it from living beings directly. The latter are called daemons, the former fae. Nearly all take on a role that leads to involvement with humans*, since having some connection with them makes it easier to channel spirit.

Roles include local spirit, imaginary friend, guardian angel, muse, psychopomp, and many more. True succubi and incubi are mercurials, but vampires?, who are still corporeal, are not.

Mercurials originate from a parallel plane loosely connected to "earth's", and many live there, visiting "earth" only for work; others live in the normal world, or pocket dimensions closer to it.

It is possible for a human to become a mercurial, but the author has not yet made up her mind how.

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*The mercurials were originally made up for a modern alternative earth setting, where non-human sentinents are so rare they are in this context negligible.

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Mercurial books show up as whatever language the reader is familiar with, yes?

Yes.

But does that mean everything in them is understandable to whoever reads them - i.e. does the language become however simple the reader needs it ("see-through" vs "translucent")?

I'd tend to level of complexity staying about the same. Particularly, footnotes referring to a different book will not suddenly be replaced with definitions of certain technical terms.

Following on from that... would someone very knowledgeable about the book's subject find more detailed information in the book than a complete novice at the subject? How about someone intelligent vs. someone stupid? (You may guess at this point why I'm curious. :) --Mutt

The information itself doesn't change, so there would be no effect of familiarity or intelligence you wouldn't get with any normal book.
If Shade did develop methods of travelling between universes comparable to the Mercurial methods, the maps would be usable, but there should be at best vague hints about the basics - that's the only reason why Nico could get away with having them smuggled out of where they belong.

I have no idea what mercurial methods are like. Do they use wormholes?
And, actually, I was wondering about the two copies of 'How To Psychopomp'. I'd forgotten/didn't know about the maps.

*adds:* Hmm, looking more closely, I probably shouldn't allow Swiffy to have this book in strict continuity...