One of the common problems I encounter in the church is the frustration many experience concerning the lack of feeling in their faith. They say they are cold, that God seems distant, that they no longer have the joy in Him they once had. I agree with what Jeremy Walker says on the subject in his book, The Brokenhearted Evangelist.
If we would have holy emotions genuinely stirred up---a freshly accurate sense of sin, a more profound awareness of forgiveness, a deeper grasp on the love of God toward sinful men, with a corresponding joy in God's grace in Christ---then we must meditate upon those central truths of God's character revealed in the being and doing of His Son, the Lord Christ. Then we might anticipate that our joy will increase. - Jeremy Walker, The Brokenhearted Evangelist
I have seen this to be true myself. I find that I am "cold" when my praying is infrequent, and my meditation is shallow. That joy and zeal fade when I am not in real communion with Jesus--the person. The more I dwell on the love of God toward me as a sinner in the person and work of Jesus Christ, the deeper I dive into all the grace and mercy I have in him, the more my heart swells with affection and the more I am moved to action.
Just two books that have been a help to me in this are The Suffering Savior, by F. W. Krummacher, and Seeing and Savoring Jesus, by John Piper. There are many others, so why don't you share any books you have found to be helpful to your heart in the comments below.