With all the books available on theology and all the commentaries out there it can be hard to tell what they are all about easily. With a good systematic theology book you just go to a subject and read the view. It's like an encyclopedia of theological points. Want to know about angels, the atonement, common grace and church government all in one place, besides the Bible, then you can use a systematic theology book to find a bit about them all.
There are several famous systematic theology books from old to almost brand new. Our generations are probably most familiar with Wayne Grudem's, Charles Hodge's and a few others but there are many out there.
A quick example and except from Grudem's work:
Chapter 34 - Regeneration
What does it mean to be born again?
Explanation and Scriptural Basis
We may define regeneration as follows: Regeneration is a secret act of God in which he imparts new spiritual life to us. This is sometimes called "being born again" (Using language from John 3:3-8)