No particular order. Each books demonstrate and deliver different purposes. The books are pretty easy to read and understand.
1. What is the Gospel? by Greg Gilbert
Understanding and knowing what the Gospel is, is perhaps the most important doctrine for the Christian faith. It's a simple question but a question not many Christians know; even, within the church. I think Gilbert fantastically delivers the definition and message of what the Gospel is. If you're unsure about the Gospel, read this book. (Also read Romans 1-4 haha) It's super short - around 120 pgs.
2. Early Christian Martyr Stories: An Evangelical Intro with New Translations by Bryan M. Litfin
Also an easy read - informative and inspiring. There are many early Christian church history books (most are known for it's lengthiness and difficulty haha), and this book is one of the few out there that's easy enough to read (PTL!) ECMS helped me redefine what it truly meant to be a martyr. The book contains several personal narratives, in which I believe it's a powerful instrument for teaching - understanding information conveyed and extracting forming characters. After reading the book, it left me inspired, encouraged, and motivated. I cried several times. About 180 pgs
3. When People are Big and God is Small by Edward T. Welch.
I love Welch - one of the counselor, speaker, and faculty member at CCEF (Christian Counseling & Educational Foundation). He does an excellent job specifically objectifying the idol of "people" - self-ego, self-esteem, attention, popularity, peer pressure, etc. It's very biblical and practical; again, very easy enough for people of all ages to read. I recommend this book to those who struggle with self-image, peer pressure, or attention. About 240 pgs.
4. Walking with God through Pain and Suffering by Timothy Keller.
Are you struggling with past pains or any type of sufferings? Are you perhaps wanting to avoid or overcome any of these? This book might be for you. I know for myself, I had a selfish and limited view of pain and suffering. Never intended it to be useful or developing and who doesn't like suffering? The book helped me identify my weaknesses and idols in my life, and redefine what "running the race" practically meant. Recognizing your pains and sufferings will help you grow deeper in the Gospel and know that it's a good thing. About 360 pgs.
5. Slave: The Hidden Truth About Your Identity in Christ by John MacArthur
Slavery was created and originated from human sinfulness but God graciously reconciled it through the gospel message. Not that I'm saying slavery is a good thing now or before, but knowing the biblical etymology can be much appreciated. Although, slavery brings about negative connotation, God draws a beautiful picture of our relationship with Christ using the image of slavery. About 300 pgs