This really is "The String Book." It is a fun, mini-article-based reader-friendly encyclopedia centered entirely on diverse string-based topics, including: knot tying, string production, legends based on string, hot air balloons, string theory, yoyos etc.
You can read it straight through or keep it as a coffee table reader to pick up and glance at for an entertaining factoid. It would make a great gift for your friend who's really into string.
The articles are short (1-3 paragraphs) and contain helpful illustrations as needed, such as in the knot-tying section. The level of detail was usually enough to keep my interest and not over-complicated.
With that said, I received this book as part of a Goodreads giveaway and otherwise would not have bought a book about string for myself. I see this as a good book on an obscure topic that it can't hurt to learn more about, but that I didn't really need to know more about.