Knot-tying is integral to stagecraft: arguably every single department has a use for some degree of knot-tying. I would actually suggest that a working knowledge of knots is useful for everyone, and ropework is a brilliant artform in its own right. From keeping things secure to making gorgeous designs, knots are a tremendously useful skill that are easy to learn and practice: you just need something to tie with and some downtime.
Geoffrey Budworth is an author of books on knots, and a very good one. His books are clear, easy to follow and all provide a wide variety of interesting knots. His terminology is clear and standardized - very important in talking about anything - and the illustrations well laid out. The narratives are also engaging: ropework has a long and fascinating history. His books are a great gateway into a useful skill and an affordable and rewarding pastime.
I would start with Budworth’s The Ultimate Encyclopedia of Knots and Ropework: it’s fun, and is a very good introduction to the topic. It covers all the knots you’ll use as a stagehand, and plenty of other useful and decorative knots, as well as providing an excellent background to ropework.
Some people knit. Others play games on their phones. I like to tie knots. Try it out.