The Phoenix Project has been very influential on me in my security career, at least partially because I share the name of the ineffectual CISO and want so desperately to avoid the link.

I think the book is still very applicable, and every security practitioner needs to be hit over the head with it (or at least The DevOps Handbook or Accelerate). Security generally is decades behind engineering operations, even though security is basically just a more paranoid lens for doing engineering ops; the ideas from Phoenix are still depressingly revolutionary in my field.