I see you are quite as shocked as I was. My first thought was that this creature had stolen the ring from Mr. Baggins and in this short and dreadful moment I could not decide which I feared more: that the creature would escape from me or that I would catch it and circumstances would force me to carry that ring myself. But rest assured, to the best of my knowledge the ring is still in Bag End.
While I stared at the location where the creature just had vanished, I began to notice a kind of rift in the air that seemed to be generated by all those books. Despite seemingly being caused by words, it is hard to explain in any language.
When I examined the spot I found that there was somehow, in addition to the usual four directions, a fifth. I may still have missed it, as the rows of shelves looked quite the same, but looking at the very direction I saw the creature again, while it was not to be seen when I was looking along the same aisle from a slightly different point of view.
The creature was strolling leisurely away. Apparently it did not expect me to be able to follow. So I made one step into this special direction.
Just one small step, but somehow it seemed immense at the same time. It brought back memories so old, that I had not thought of them any time since the arrival of the Eldar.
You know that I was not born within Eä, the World that Is, or Arda as is now called. In fact I had my part in creating it with the Song and no matter what happened later on, no other experience was as formidable as entering this world. When I entered, I had taken it for granted, that leaving would not be possible until Eru lets the world come to its end. But that was exactly what I had done.
Now as soon as I stepped through the rift, the creature spotted me, but while I was shaken by the knowledge of having left Arda,the creature was frozen by surprise too. And as soon as we both had recovered, the chase began.
Its legs were obviously not made for running, so keeping up with it was not hard. However this space, whatever it was, grew stranger and more confusing, the further we went. In some places there were not only five, but seven or eight different directions and even up and down were not always what they should have been. Following it was much more a contest of the mind than of the feet.
No elf would have been able to follow, nor do I think a human, though if any of Eru's children will some day be able to walk these strange ways, it will likely be one of the younger children. Their ability to quickly adapt to new situations is admirable.
That I was able to follow had a lot to do with the shock I felt when entering this space. The fact that there are up and down, back and forth, left and right, had to be learned when we came to Arda and so I was just in the same situation again: having to learn the rules by observation. And here I had a teacher, however unwilling it was.
When I thought I would catch up with it any moment, the creature showed me what its body really was good for. We were in a big hall and in the centre there was a spiral staircase leading to some upper floor. This stairs wound around a pillar with nothing to support them on the outside. Although they missed the elegance of an elfish design, they looked frail, but proved to be quite robust.
Instead of walking up the winding stairs the creature just climbed the outside, going from one handrail to the next with an incredible speed. While I went up the stairs in a desperate hurry, in the back of my mind I noticed that the creature must be a tree living animal. Upon reaching the top I barely saw the creature turn a corner, but it was enough to follow.
Again I drew nearer and nearer until I suddenly noticed one book.
During the pursuit I had already passed more books then I had seen in my whole live before, but so far I had not paid attention to any specific one. Whether it was a kind of magic or fate, I did notice this one and when I managed to release my eyes again, the creature was gone.
The title of the book was The Silmarillion.