This blogpost assumes a few things: that you preach, that you use the Bible when you preach, and that you preach through whole books of the Bible. If all of those apply to you, then perhaps this post will be helpful.
Below I have listed a few thoughtful considerations when choosing the next book from which I will preach:
- Which books have I already covered in my expositions? This is important because it might show you the heaviest types of books you have emphasized in your preaching thus far. Perhaps you have favored the Gospels or Pauline Epistles in your preaching. Maybe you are about to finish preaching through a large book, like Psalms or Genesis. Maybe you will notice that you have preached exclusively in the Old Testament or the New Testament. Maybe you have preached one letter out of a series (1 Corinthians, 1Peter or 1 John for instance), and have not covered the other epistles. By asking this question you will have a better grasp of any places you need to avoid and others you need to seriously consider.
- What are the major needs of my church right now? If you have many newer believers, the Gospels would serve them well. If you have a church that is more spiritually mature, but could use some teaching in deeper doctrine, like soteriology, perhaps Romans or Ephesians would be a good pick. Maybe your congregation has been beat up spiritually, or they have just gone through a major upheaval or split. Which books would help bring healing and peace to their souls during this time? Along with their spiritual needs, you might consider the needs of the church body in practical matters. Are you seeing many families come in who could benefit on teaching about God’s plan for the family? Is your church financially wealthy and they need to see how God would have them use their wealth and spiritual gifts for the glory of God? Many times pastors think that to answer these practical types of questions they need to go to a topical sermon series to answer these needs, but a well chosen book can help while at the same time demonstrating the way that the Bible is to be read as the Word of God and not a “fix-it manual.”
- Where have I as a pastor avoided preaching because of my own weaknesses? Pastor, never forget that you are not just a shepherd, but a lamb, and you need to grow as well. Sometimes we pick what we know and are most comfortable in regard to books and subjects. Do you struggle with the discipline required to stick with preaching longer books—choose a longer book and preach it! Do you drag your church through minutiae in the text? Preach an overview of a book in a few short weeks. Do you struggle with your understanding of the Old Testament practices of Israel and how they relate to the Gospel? Preach through the book of Hebrews. Do you love long books? Preach through a few short books. Preach almost exclusively from the New Testament? Preach through an Old Testament book. If you only preach on subjects and from books from which you are comfortable, then you will not grow in your knowledge of the Word, but worse, you and your church will become anemic in the areas you fail to preach.
- Where do I see the church needing to go in the near future?Are you needing to develop deacons and elders in your church? Prepare their hearts to understand what the biblical requirements of these offices are by teaching them from the Pastoral Epistles. Are you seeing that your church has lost its heart for evangelism? Think about why that is, and then prepare to take them through a book like Acts, or the Gospels. I think it is always a good time to point out to the Church what it is like to live in a society that is hostile toward God, his Word and his people. This will prepare them to trust in God when times are bleak and will help them endure during persecution. The books of Daniel, Acts and Revelation are great for this purpose.
While it is true that the Spirit of God will use whichever book we choose to preach from, we should be more methodical and thoughtful as we prepare for preaching through a new book. This means we should begin with prayer, think thoughtfully about questions like those I pose above, and then use wisdom, depend upon the Lord that he will bless the preaching of his Word to your Church.
Always Reforming 
Those are helpful questions to think through. What if there are multiple issues, needs, or weaknesses that you see? How would you evaluate and respond? How would you practice spiritual triage with regards to book selection?
Great questions! When you have a good idea what the issues are that need to be addressed, that might help point to a particular book over another one. For instance, if you need to address the doctrine of salvation, the issues of the family and perhaps a struggle among some who are not getting along with one another, Ephesians would be a better pick over say, 1 Timothy (not much about family or loving others) or 1 Corinthians (not a lot there regarding the doctrine of salvation). The good thing about preaching through a whole book is that 1) it meets spiritual needs that perhaps you don’t even know about because they are largely hidden issues of the heart and 2) they will prevent any person from becoming defensive about your subjects because you are simply preaching through a book and not targeting an individuals pet sins and 3) every book deals with several subjects, so even though you might choose a book for a particular need(s), you will soon move on in a healthy way to address other spiritual needs of the greater church so that your preaching does not become tedious or overbearing. Hope that helps!