"Identify bottlenecks" - I hope this is the first topic covered. There's no point in fiddling about with PHP if the actual bottleneck is the fact that you're sharing a slow connection with dozens of other companies or are hosted on the opposite side of the world to most of your customers.
Having said that, I'm looking forward to the book, as there doesn't seem to have been much on this topic since the O'Reilly books on scaling web applications (2006 I think).
Not sure about 'best', but the way I go about it is to measure the total time required for an application to complete a task, then attempt to measure the individual components (time spent in PHP, data transfer, rending on client side etc.) and keep narrowing it down until you find the exact cause, such as a database query which requires optimisation.
Having said that, I'm looking forward to the book, as there doesn't seem to have been much on this topic since the O'Reilly books on scaling web applications (2006 I think).