In the Earth Section of Zathroth's Secret Library we can find 3 types of creatures: Cursed Books, Ink Blobs and Biting Books. The first creature is unique to this location and has a book found on the shelves in this room (here) that explains its origin. Although we are talking about the earth section, it is also important to know the other books that exist in the ice and energy sections (the fire section I have not found any information in books to date, although I have searched), I will leave out the explanation of other creatures that are not books because they are not directly or indirectly linked to the question of this topic. I will leave here the reference source directly from the official site in the bottom of the answer.
Well, directly an introductory explanation about the origin of the other powerful books in this library: Icecold Book and Energetic Book.
Origin of Icecold Books
Icecold books were created by the race called cryomancers, they became extinct and these books continued to exist in inert form, were stolen and experimented on by those who craved the knowledge contained in their pages (in this case they were taken to Zathroth's Secret Library). The way found by Zathroth and his followers to release the power of these books was to give them a life of their own with energy maintained by magical ice crystals.
Origin of Energetic Books
The energetic books were actually a race of shapechanging demons who planned to infiltrate the Secret Library to steal enough knowledge to defeat the Prince Demons and thus take control and dominion over other demons. Their plan was discovered by the librarians, they justified that they were in the service of Zathroth but the verdict of this god was to permanently transform such demons into the books they were trying to steal.
Origin of Cursed Books (They are exclusive to Earth Section of the Secret Library).
Detailed explanation about the book about the Cursed Books, with excerpts and comments about each part of the book (it is also on the official site):
Though some potent books can gain consciousness and sentience on their own, this fiendish creature is created by botched attempts to read them and harness their power.
The book makes it clear that some of the powerful books present in all parts of the Library can gain consciousness on their own, in this case an example is the Icecold book by means of the magical ice crystals. But this creature (the cursed book) is created by unsuccessful attempts to read and control the power of the powerful books.
Sometimes wannabe magicians try their luck on spells way out of their league with typically dramatic consequences. Most such misfortunes lead to the untimely death of the vain caster in spectacular fashion, sometimes, however, it happens that some lesser demon is partly summoned and melts into the book containing the spell by absorbing the life force of the unfortunate reader.
This part of the text mentions the forms of creation that are nothing more than failed attempts by magicians to cast spells beyond their control (contained in these powerful books), which lead to the death of the conjurer in spectacular (explosive?) form, and in other cases the death of the conjurer produces a demon of lesser strength that is summoned and fused into the book.
Such demon-book conglomerates can become enormously powerful by harnessing the knowledge contained in the book, demonic forces and whatever might be left of the casters memories and fractured soul.
This fusion of the demon originating from the conjurer's soul and the book produces a powerful entity known as the cursed book, which draws on the power contained in the book, the demonic powers of the creature momentarily summoned, and the memories of the deceased conjurer.
For their raw power and potential but also for their contained knowledge, such cursed books are hunted down by Zathroths librarians and bookkeepers and dragged away to some sinister place of forbidden knowledge. There they are trained, their skills honed and they become part of Zathroth's collection and serve as his guardian dogs to protect other secrets.
These powerful books contain knowledge of interest to Zathroth, for this reason they were taken to the Secret Library, there they were trained and converted into servants of that god, their objective: to protect the secrets of that place like a guard dog.
Perhaps because of the way of their unwilling creation, cursed books feel nothing but spite, contempt and hatred against living beings. Sometimes they even plot to incite mortals to follow their own foolishness and use spells beyond their power. Whether they do so to create more of their kind or just to enjoy the misery of others can't be told for sure. Most of them just skip such subtlety and attack anything not allied to them or their masters with all of the raw power available to them.
Cursed books hate living beings and incite mortals to try to conjure spells beyond their power, one of the possible explanations for this behavior is the goal to create more cursed books or just to amuse themselves by seeing the misfortune of others. Some of them are so uncontrollable that they attack even their masters.
Conclusion on cursed books: Cursed books are created in a tragic way, with the death of a mage by conjuring an extremely powerful spell that is beyond his power, such an action can generate a spectacular death (and generate nothing with it) or create a cursed book by absorbing the soul of the conjurer. These books were collected by the servants of Zathroth and converted into guard dogs, their ultimate goal: to protect secrets. Cursed books create other cursed books, this means that in the place where they are found possibly many other mages have died in both possible ways (the spectacular one or the one that produces the absorption of the soul and creation of another cursed book), such a form of spectacular death is not fully explained but an interpretation is something of explosive kind, and this explains the damage and repairs in this area of the library.
About the other creatures on the Earth Section, a summary obtained from books and the official website:
These are creatures created from nightmares (Roshamuul origin?), they were created by Zathroth's librarians after several experiments. This is not the first version, but it was the one that best suited its function: guard-dog of secrets of that place.
Certain books overloaded with energy (which in many cases is derived and absorbed from their creator) can "bleed" their excess life energy in the form of magic ink, a single overloaded book may produce only a few drops of such ink, but a large number of books may be enough to produce veritable ink blobs. This accumulation of magic ink takes on strength and life of its own, seeking more and more ink to feed on and grow. Theoretically they can grow infinitely, but whenever one reaches an excessive size a librarian splits it into 2 smaller bubbles for better and easier control. Their goal: to scare off intruders.
Conclusion about the origin and function of all the creatures in the location and the location itself: They are in this location to protect against intruders, their goal together is to prevent secrets from being discovered in this location. The dramatic form of cursed book creation may explain why the place is much smaller than the other sections, the construction repairs in the place can be explained by spectacular and possibly explosive forms of unsuccessful spells of mages trying to read the books in that place.
The question that remains about this place: what is different and should be protected by these creatures?
Besides practically empty rooms and a few books explaining the origin of the creatures in the library (with the exception of creatures from the fire section), there are 3 areas that draw attention by contrast to the library environment:
Area with sand and plants from tropical beaches, a sign written Dark - Desert. With emphasis on the two initial letters (Da/De).
Area with forest and typical forest plants, a sign written Jacket - Jungle. Highlighting the two initial letters (Ja/Ju).
Area with snow and ice, a sign with Snow - Svargrond. Highlighting the two initial letters (Sn/Sv).
Do these letters form an anagram? Possibly, but after many tests I came up with nothing. Although there are no explanations in books on the site, the presence of sand, plants, grass, snow and ice may be due to powerful spells that failed and killed the conjurers who craved great power. This area is small and there is little material about it, I have tried to condense all the material I could find on the subject, I hope you have a good read!
All the sources can also be found in this room of the Earth Section (this room).