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
How to Choose the Right Bookbinding Thread
Linen Bookbinding Thread - Book Craft Supply
Binding Thread
Hand-Poured and Molded Beeswax Thread Waxers
Bookbinding Sewing Thread Packs – Traditional BookBinding
35/3 Linen Thread - John Neal Books
DIY Tutorial: How to Choose the Right Bookbinding Thread
Bookbinding Supplies Beaucoup! — Bari Zaki Studio
Bookbinding Models
CNBTR round Waxed Thread for Leather Sewing - Leather Thread Wax String for Leat
3 Rolls of Sewing Thread Bookbinding Cord Cobbler Thread Tailoring Line (Black, White, Brown), Size: 5.91 x 1.97 x 2.36
Leather Sewing Waxed Thread Sewing Tools Accessory - 300m Thread Leather Sewing - Aliexpress