I could never say enough about this book. It was recommended to me last year by my spiritual father.
Fr. John is a monk who lived in the Valamo monastery and then New Valamo in Finland; he died in 1958. This book is a compilation of letters he wrote to his spiritual children. It is very accessible and has deep wisdom. In many ways, this book has and still is showing me what it means to be a Christian, specifically as an Orthodox Christian.
I wish I could explain what this book has meant to me, but I find words rather absent. Suffice to say that this book opened itself to me, but that will take the rest of my life to go towards what it teaches.
Here is one quotation:
No, we must not trust ourselves before we are lying in our graves, and whether we persevere in virtue depends not on us, but on the grace of God. The Lord preserves the humble; in so far as a man humbles himself, he flourishes in the spiritual life....
-from letter 79
It is published by Saint Vladimir's Seminary Press and is still in print, with a slightly changed cover.