Since it is time for the apostles fast, many priests recommend reading the book of Acts in the Bible throughout the fast.
Another great book for this fast is "Timothy: Paul's Closest Associate (Paul's Social Network-Brother and Sisters in Faith series)" by Bruce J. Malina. If you are interested in knowing about the people who were involved in spreading christianity, then this is the book for you.
"The Life: The Orthodox Doctrine of Salvation " by Clark Carlton is a great book about salvation and the orthodox faith. It also addresses several issues that have to do with the doctrine of salvation. Even though the doctrine of salvation is sometimes complex to understand, the author puts it in an easy, and simple formate to help reader better understand the concept.