Books are traditionally sewn with linen thread which is very strong and can be purchased in several sizes.

Choosing the right thread is important for a successful repair. Using thread that is too thick can damage a book because the text block may not fit back in the original case. Thread that is too thin can tear through the folded signature paper. In general, it’s best to use as thin a thread as possible, but choosing the right thread depends on the kind of paper and how many signatures are being resewn.

LINEN THREAD SIZES
Common sizes of linen thread are 12/4,18/3,25/3,35/3 and 60/3. The first number is the thickness of the strand or ply and the second number is the number of strands or ply per thread. So 12/4 is composed of 4 strands that are 12 thick. The 12 refers to an industry designation that relates to the weight of the wholesale quantities of thread. In this sizing system, the smaller the number, the thicker the thread so 12 thread is thicker than 60 thread.
35/3,30/5, and 18/3 are a good assortment of threads to have on hand. It can be hard to recognize a particular thread size so it’s a good idea to mark the size of thread on the inner cardboard spool before discarding the paper wrapper.