To reiterate what I've taught and been taught, when you are binding books you generally need a thread that will not stretch or break
German Linen Thread 25/3 50 grams – Hollander's
DIY Tutorial: How to Choose the Right Bookbinding Thread
Bookbinding Supplies Beaucoup! — Bari Zaki Studio
6″ Anselm Bookbinding Kit – Peg and Awl
Waxed Bookbinding Thread in a wide variety of colours and available to purchase in small, affordable quantities.
Waxed Bookbinding Thread - 6m
Trying Top Rated Bookbinding Thread
Set Of 16 Bookbinding Kit Tools Set Bone Folder Paper Creaser, Waxed Thread, Awl, Large-eye Needles For Diy Bookbinding Crafts And Sewing
Best Waxed Thread –
Leather Sewing Waxed Thread Sewing Tools Accessory - 300m Thread Leather Sewing - Aliexpress
50m red bookbinding thread wound onto card. note : this is more of a burgundy , not a bright red. Excellent quality, lightly waxed, cotton and
50m Strong Red Bookbinding thread, slipping twine