The first volume of Saint Josemaria's letters (Cartas 1) has just been published as part of the Complete Works of St. Josemaria (Rialp). It contains his four earliest letters. They are helpful to deepen one's knowledge of the spirit of Opus Dei. If you read Spanish, you are encouraged to read it and use it for your prayer. The ebook can be purchased for Kindle. The English translation is being prepared, even though it is not known when it will be available.
The book can be ordered through Scepter at usamadrid.com or scepterespanol.org.