The book is, as mentioned, mostly about the elemental planes. The Elemental Plane of Fire and the Abyssal Plane both sound incredibly dangerous, though! In fact it's not recommended for anyone to try exploring the Abyssal Plane at all unless they're an Adept with decades of experience projecting their mind to the other planes, and have a guide who's been to the Abyssal Plane before. 

In general, the plane of Earth is safest, followed by Air and then Water. (None of them would support human life if you were to literally transport your body there; their physical laws are believed to be different from in the material plane, and certainly magic behaves differently.) There are instructions given for mind-shielding techniques. 

What exactly happens? Fire isn't dangerous to look at or think about, it's dangerous to touch, so if you aren't physically there what goes wrong?

The 'fire' in the Elemental Plane of Fire isn't exactly the same thing that ordinary fire is, but in any case that's not the dangerous part in itself. The main risks are, one, the ambient density of magical energy is several hundred times higher there, and a mage projecting there is, by default, opening a channel between that and themselves – if they're not careful, the energy will follow the gradient 'downhill', just like when tapping a node, except intensely enough to seriously damage mage-channels. 

The other risk is the local life. The more intelligent Fire elementals can perceive the mage-energy signature of human visitors, and will tend to go investigate. And, being native to such a magic-dense place, they're absurdly powerful and can accidentally cause serious magical injuries just by trying to communicate. (And sometimes it's worse, and they're hostile...) 

The demons native to the Abyssal Plane are the main danger there. And, unsurprisingly, they're a lot more dangerous in their home environment than when they're summoned to the material plane in construct-bodies. 

Huh! Does this book say anything about the spirit world at all?

It's mentioned a few times – it's in the list of planes that can be visited by mind-projection, and in the section on existing and historical practices around extraplanar projection, where it's specified that this is a skill priests and shamans of some religions pick up. It's rated as somewhat less dangerous than the Elemental Plane of Water, but more dangerous than Air and Earth. 

"I guess that does seem encouraging but if part of the problem is hostile locals it might not be uniformly safe-as-these-go," she remarks.

"Oh! I forgot to say - I asked the librarian, and there's a class on extraplanar projection! It's an advanced one but I might be able to get permission to take it next session anyway. If I'm properly trained on the elemental planes then it feels more like I can judge how dangerous the spirit world actually is." 

"Oh, cool! Maybe I'll take it too - depends what it's competing with, I don't feel very urgent about it."

"Neither do I, I guess, but I don't have that much on my list of classes I was excited to take next." 

"I wanna learn to summon elementals, even though that adds a language course too."

"It seems really useful! You could ask them questions about how things work in the other planes..." 

"Yeah! And probably some of them also want to know about us."

Nod. "Probably! Maybe you can take the summoning class and I'll take the projection class and then we can teach each other? Anyway, this is the most on-topic book I found - there's another one about historical magical accidents that mentions a few mages who got killed doing projection to the Abyssal Plane or the plane of Fire. No one's listed who died going to the spirit world, but it could just be that usually that's done by priests or shamans and they get taught properly - oh, and probably aren't trying to do it for combat, the thing that got these mages killed was deliberately trying to track down hostile natives, in order to try to put bindings on them and use them as weapons." 

"Wow, rude."

"Also seems really stupid!" 

"- is it? What goes wrong apart from it being kidnapping?"

"Well, if you run into one that's more powerful than you were expecting, or if the extraplanar beings catch onto what you're doing and start teaming up against you - some are smart enough to do that - then they can get past even very good mage-shields. The book did say that no one's ever died projecting to the plane of Fire if they had proper training for it and weren't looking for trouble on purpose." 

"I wonder if anybody's being posthumously accused of looking for trouble when they weren't really."

"...Maybe. The case studies in the book seemed convincing, for all of them it wasn't the first time they'd gone looking to bind powerful extraplanar beings. But there could be others that aren't in the book because they weren't well documented or whatever." 

"It's still encouraging."

"And I'm hoping whoever teaches the class on it will know more about it. Anyway. I think this is probably everything we can find on this for now - did you still want to go try talking to other temples, too? I looked up and there's a few that aren't too far away." 

"Sure, which ones?"

Ma'ar digs out some notes. "There's a temple to Vkandis that's kind of far, but I can go on the day I don't have any classes. And then there's a temple of the Twain which is a lot closer, and a small one for - Bestet the Battle-Goddess? And also the Nameless God of Eternal Flame, but that one's way at the edge of the city." 

"I can get Skan to fly me if it's far to walk but not too far to fly with a passenger."

"Oh, right! Maybe you can do the Eternal Flame one then, if it's in his flying range - I think it's six or seven miles?"