Final answer:
Sophie's textbooks were tied together using a ribbon made of silk, as indicated by the reference to winding blue silk in the provided text.
Step-by-step explanation:
The ribbon tying Sophie's textbooks together was made of silk. This is derived from the provided text that mentions the daughter of the Professor winding blue silk on a reel. Other historical methods of binding, as in the case of bamboo strips from ancient China, involved cords or threads.
However, in the provided contexts, silk is explicitly mentioned as a material used by a character in relation to books, suggesting a continuity in the use of textiles in book-related applications, from the bindings of ancient texts to the ribbons used by Sophie.
This echo of textile use across history underscored by themes from the Bauhaus movement, which integrated artistic design and textiles, further indicates the importance of materials like silk in various forms of craftsmanship, including bookbinding.