Trying to find any sense in what the Devs say.

Tefusanglings and marfusangs have no models of their own. They are the same tefusangs all over again, rescaled and repainted somewhat. No backstory.
Four species of arangmas share the same model, they are just colored differently. No backstory either.
Things like that make the game look half-baked, and the team lacks resources to fix the situation. No one plays a tefusangling or an arangma.
Are they going to be flushed down the toilet? No. Not that we know of.

Something very wrong is going on. I'm not especially happy to side with Aiwendil on that, but it seems there's no other choice left to the players. "They" don't respect neither the world itself, nor our choices, so why should "we" respect the choices "they" make? Respect the true Settings instead, play your Diaboli according to what was created by the previous Devs team, the one that made the world and is long gone by now. In some cases, gone rather unwillingly.
What could "they" possibly do to stop that from happening? Take the Diaboli model away? Update the main site? Start banning role players for "bad" descriptions? Oh yeah. *crying out loud*

Use the same Ynnwn model for the Diaboli, just make their skin color darker.

@bilbous. The only 'book' not created by myself that I have destroyed so far -- and quite recently, too -- was some brutal IC rant against Laanx' followers, placed directly on Sharven's face. He just stood there, having a book instead of a face. GM repercussions or not, I would readily do the same thing again if it happened again.
In any other case, and especially with OOC stuff involved, I think it is much better to ask for the rules in action, and then abide by such rules no matter what.

Sarva, so is it okay to advertize things, in the form of 'books', directly on the mines? Or not?

@Candy. Thanks a lot for your contribution, but it seems you didn't get my point. I'm not interested to know if other players can interpret it all IC'ly or not. I want to know if they can do the same or not.
Maybe the mines of Yliakum would look better with a forest of imagined signposts growing right on them, I don't know... tastes differ.

If it's not littering, is okay with the rules and is considered fair competition, why would I -- or any other player -- sell or otherwise destroy them?

I'm just interested to know if it's okay with the rules. When I started playing, I was told -- or was it in the Rules section somewhere? -- that players can only leave objects on the ground for a limited amount of time, and only if their RP requires it. But at the moment nearly every mine is 'marked' with OOCish trade ads of a certain guild, which makes me wonder about the rules or guidelines probably being changed somewhere along the way.
If the old restrictions do not apply anymore, I hope everyone who is buying ore (including me) is allowed to spawn and spread multiple 'books' in the wilds to get even with those who do.
It would certainly make the mines look nicer and the world more believable