Armies and military forces have handbooks on preparations, best practices, and tactics for warfare. The Philokalia could be characterized as a handbook for spiritual warfare. It's written by generations of monks and ascetics, who learned from their masters, who, in turn, became masters themselves. As such, there is a timelessness about it --- it's a distillation of the wisdom of the ages.
The principles in the Philokalia are applicable also to ordinary Christians, not just Christian ascetics --- how to stay focused on the things of God, how to build a life of Christian character, how to deal with temptation, how to restrain the passions and redirect those, along with the intellect, into greater devotion to God and the Church. I found its principles instructive and directly applicable to my Christian walk -- a worthy guide to me as I begin this phase of my Christian walk with my recent conversion to Orthodox Christianity.
Given the stereotype of ascetics such as monks and hermits as being out of touch or having their minds on some alternate plain of reality, from reading the Philokalia, it's clear that such is not the case. I do not think that someone could put one over on the authors in this work ---- they show an keen, realistic insight on human nature, and then show wonderful insights on how to take this willful, self-indulgent nature and abnegate in favor of something far better --- a relationship with Jesus Christ.
I very much enjoyed reading this book, and it will be a treasured addition to my library that I will reference and re-read time and again.