The most common book new believers are encouraged to study is the Gospel of John. The problem with that is that new believers can quickly become confused and discouraged by taking the promises made in John and applying them to their lives.
The book that best establishes believers in the faith is the book of Romans. Romans begins by laying out the gospel of the grace of God. From there it explains the believer’s position in Christ and how that position teaches us to walk without a standard of ordinances. Then it explains the dispensational aspect of where we are today and who we are not. Finally, it shows examples of how to put all of this into practice.