A great resource on flexibility in general, including a wealth of dynamic and active stretching techniques, with clear pictures and descriptions illustrating each exercise.
The book is geared mostly to martial artists, but the flexibility work included can be used for dance (and life!) as well.
Note: I already knew (and apply!) many of the book’s lower body stretches from taekwondo and ballet, but I still found some very interesting and challenging progressions as well as a few easier variations to play with in each of the chapters.
I use this book mostly for inspiration on upper body and torso flexibility work, as this way it complements the more leg, hips and torso-focused exercises from Stacey Nemours online course package for dancers very nicely.