Any strong, thin, thread is suitable for bookbinding and similar crafts. There is a wide variety of artificial and natural threads, with different properties. I cannot write anything about the various, excellent, threads beyond suggesting that if a reel of thread is already in your hobby box check it for strength and enjoy using it.
Modern threads rend to be made from continuous filaments that are squeezed like very thin toothpaste from a tank of plasticy chemical. It quickly hardens, and is wound onto spools of appropriate size. If just one filament is made it is called a monofilament. Examples are fishing lines, strimmer machine cutters. They are strong flexible, and very slippery when it comes to tying knots in them. Fishermen have a variety of clever knots that overcome this.
Traditionally, bookbinding threads are made from flax fibres processed into linen threads. Cotton is also used by threadmakers, but the flax is stronger and with a longer staple than the fibres from the cotton plant.
Linen thread is widely available from good haberdashers, often called buttonhole or carpet thread. Bookbinding suppliers usually have a wide range of thicknesses and colours available. Washed and unbleached linen is a pale yellow. It is sold by large spools (cops), smaller spools, hanks (skeins), and cut lengths.
It helps to understand how thread is made. Imagine picking up a handful of grass cuttings after mowing the lawn. Lay the individual bits of grass end to end in lots of side by side rows, being careful to stagger the joins of the ends. Then carefull roll all the bits of grass into one sausage. It will, should you try it, fall to bits very quickly. Twist the bundle around lots of times until the individualy blades of grass squash each other. When they are tightly compress the weakness of the between the ends of each bit of grass is diminished.
If you keep the grass rope under a slight tension it will be fit to use (in your imagination!) as thread. But As you let it go it will tend to unwind, and all the effort will be lost. Threadmakers, string makers, rope makers all take several strands and twist them around each other but in the opposite direction. In this way the two different twists combine to make everything more stable. A useful rope is formed.
In order to help you visualise what was being done, I used an impractical example. It is right to think of using grass mowings as being far fetched. But instead of trimming an already short lawn imagine using longer grasses, such as the ornamental ones that grow very tall. The number of weak points (where the ends are) will be greatly reduced. The length of the fibre used is called the staple. The longer the staple, the better.
Then think of the flabbiness of the grass, and that it will dry out into hay. If all the soft vegetable matter can be removed, leaving the main supporting backbones of the grass blades then the bulk of the rope, or thread, can be reduced, without affecting its strength.
Our crude example has been refined over the centuries, but linen thread is made in the same way. Flax stalks are harvested soaked and dried and then beaten so that all but the strong stringy stuff remains. This is then lined up (carding) and spun, then counter spun with several other strands. That is how linen thread is made.
Fibres from cotton plants, animal fur (notably sheep fleece), hemp, coconut, all have to go through the same process making thread, twine, knitting wool, rope, cordage, yarn, string.And silk.
Artifical variants have the advantage
