I never knew what true religion was till God sent me this excellent treatise. –George Whitefield
Amazon continues to make the transition from a print to an electronic library cost-effective. Today you can pick up Henry Scougal’s classic The Life of God in the Soul of Man for just 99 cents. It’s an old classic, not written in twenty-first century prose. But if you’re willing to read a challenging (albeit brief) volume, you’ll be richly rewarded.
Since it’s in public domain, you can also find a free pdf of the book online at various locations (e.g., here). You lose the functionality of search and highlighting, but if you don’t have an easy way to read it electronically, this might serve you well.