The secret of that empty Thaian library, with no books and no librarian, have bothered me ever since I started playing Tibia on my first account (and that was a looong time ago...)
I've been hoping to solve that mystery too, so I still check the bookshelves from time to time, just in case some new clue is added or some old one "respawns" after one of the major Tibia updates, but I've found nothing so far...
If I may share my personal theory about the subject, it almost looks like either:
1) this library was abandoned and/or removed somewhere else in a hurry (perhaps when Ferumbras attacked the city, long time ago?), or
2) it was delibreately created as some sort of diversion, to DISTRACT the common citizens (or at least some possible spies and enemies from another realm) from the REAL libraries, which are hidden in other parts of the city.
Either way, the only thing I know for sure is that there are indeed 2 other places in Thais which contain quite a large number of books, so I consider them the REAL libraries:
- One is up above the Thaian Castle, where the Royal Archives are located (that's a really interesting read, by the way, which I think provides clues to some older quests, for example about the journeys of Alister Cropwell).
- And the other library is located above the Sorcerers Guild in Thais, where npc Muriel resides and sells sorcerer spells (or sends players on a little quest in the search od Death Ring). Now the intresting thing is that ONLY SORCERERS might enter or hire an apartament there (so this library is quite well hidden, beacuse we have to play a character who is indeed a sorcerer to even enter there, and therefore not every Tibian player even knows that this kind of place exists). The other curiosity of that place is the mysterious female npc named Lungelen, who walks deep in her own thoughts among the bookshelves and doesn't even want to talk to us... (As TibiaWiki puts it "she studies the ways of the ancestors and doesn't want to be disturbed". They also claim she is the head sorcerer of the guild, even above the much more visible Muriel...)
- And finally there are some other books scattered inside or around the Thais city (for example, some hidden on MacRonalds family's farm, or some hidden in the endless tunnels of the Ancient Temple...)
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But what really happened to the main Thais library, between its temple and depot? That, as far as I know, remains one of the unsolved mysteries... So we may only speculate, as I have done it above... or we may try to find some possible clues in the books located elsewhere, inside and around the city. And me may study the lore of Thais by talking to its various npcs, of course.
I will keep my eyes and ears open for any relevant information about this subject, and if I discover something new, I'll let you know! :)
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EDIT: Oh, I've just remember something which might be of value too: the huge library on the ISLE OF KINGS! How could I even forget about that one! It is true that it's location is closer to Carlin than to Thais, and I haven't visited it for quite a long time, but I'm quite sure that the graves of the old THAIAN Kings are located there, so perhaps THIS is the place where the original Thaian library was relocated during one of the wars, to save the priceless knowledge? I think it's entirely POSSIBLE! Don't you think?