Unfortunately, you can only teleport to the one spot designated when creating the book AND back to any book from that world. More of a gate than a teleport.
What are the possibilities of the True Language? If it would be some kind of 'primary magical language', you could simply cast spells through writing.
See Goo and Ash above. But more seriously, it defines new realities. It does not allow you to change your existing reality.
Perhaps echos of this language is what allows for traditional magic. Magikal words have True Language sounds embeded in them, much the same way the books do. Thus why you have to study and meditate so much when dealing with magic.
The technique to write a book, in theory is quite simple. In practice, I could see someone taking decades to perfect this. (Given how long it takes someone to master Enochian, and the square, this is not unreasonable. In addition, if you have looked at the various bible codes.. seems easy enoguh until you try to do it.) So it may be something of a secret. There may not be a written text, it may require apprenticeship. This gives the scribes a monopoly, while they might be free with the concept behind the secret (knowing they are pretty safe, as long as one of the scribes does not defect).
If Myst (D'Ni), they say the books only open portals to the place that is closest to the world described (thus allowing for interesting complications to exist that the original author did not foresee). In Amber, the world already existed, but it was not "real" yet, until an Amberite learned of it, thought about it, and made it so. So as long as the amberite is careful they will get exactly what they wanted. If they are messy in their shifting, it is not.
Here? I like the concept that everything is equally real. The book just acts as a bridge between your real reality and the probability that this other place is. This also explains why you can't alter your own world with a book. (Either that, or someone else has your world's book).
The reason for this is the book should continue to write themselves... technically, as time advanced. If you could read the book without getting sucked, in you might even see new pages develop.
Perhaps scribes add pages to books who become difficult to link to. Or if they note changes, perhaps they add appendix's to the original books.
Or as long as the world changes along its natural paths, it does not need additions. It is only when outsiders radically change a world that additional pages are needed.
Or maybe it does not matter, as it created a link as the world was... AT THAT TIME. From there, time proceeds normally. In addition it could explain why the world is never quite what you originally described (think of it as the differences everyone has in how they see a fictional world, as they each read the same book).
I see this world as being magic rich, rather than a high magic world. What magic is there is potent, but there really isn't a lot of it (unless you are a scribe and that is one long ritual in making the item). Hubris is very possible. It depends on how you want to play or install this into your game world. They could be a humble people, knowing that they were blessed with a divine language, that they were a favored people by the diety (and could be unblessed just as easily). They could be total arrogant fools, pillaging all these "echos" because only their reality is real and these other places just imaginary. Perhaps they are secretive and cautious, as other people could just as easily link to their world. There may be a variety of opinions on the realness and uses of these worlds, so the issue might be muddled.
I would believe that scholars have a tendancy to be polite and tend to overthink issues. If they delve into philosophy too much, they could easily have an intricate ethical code about what is and is not acceptable in a true book (though not what is possible...). Insert as you see fit for your campaign.
