The subtitle of your book includes MongoDB. What guidance would you offer for choosing between an SQL and NoSQL database? For applications installed on customers hardware, there can be reluctance in larger DP shops to applications that do not use a major SQL database.
"Hell hath no limits, nor is circumscrib'd In one self-place; but where we are is hell, And where hell is, there must we ever be" --Christopher Marlowe, Doctor Faustus (v, 121-24)
In very huge generalization: When you have data with complex structure, then relational databases (SQL). When you have data that can be simply arranged as key-value, then non-relational databases (NoSQL).
In reality, you need to learn both to know the best which one to choose. When I was playing at first with NoSQL, I was thinking that moving from MySQL to Apache Cassandra will be an easy task. I couldn't be more wrong, and eventually it turned out that Cassandra is totally not a way to handle the kind of data I had.