After reading How 30 Days Without Social Media Changed My Life it provided a link to a book the author is working on titled Scaling PHP. I was so impressed by the premise of the book I purchased the alpha version of the book and it's cool because the author provided a read-only GitHub repository so we can see the book as it develops.

If you're interested in making high performance PHP sites I would recommend signing up for the email list so you know when it will come out.