The reader will greatly benefit from this book by grasping a profoundly biblical grid for understanding the purposes and benefits of speaking in tongues.
While it is a good testimony of Engelhardt's personal experiences of his faithful pursuit and service of the Lord, and I believe he is sincere, the conclusions and promises promised through the speaking in tongues as a devotional prayer language, specifically, are highly questionable.
Similarly, Engelhardt's "proof verses" for his doctrine of "speaking in tongues" as a prayer language seem to be the result of someone who had a spiritual experience and is seeking to validate it rather then someone providing sound biblical support for a spiritual practice that is clearly shown in scripture.
Engelhardt's experiences resulting primarily from praying in tongues are questionable because although he practice "tongues" he was also deeply involved in traditional, biblically sound practices of prayer (in his own language), bible study, seeking the Lord's will for his life, and service to the church throughout his Christian walk.
What Engelhardt ascribes to his 'unknown prayer language" could have equally been ascribed to his overall course of being faithful to his pursuit and service to the Lord.
It's a very good book, but the organization could be better. However I agree with the message for all but how stringently the author seems to read "private-use-only" into speaking in tongues. I understand it, but I would disagree. As a foundation for understanding this gift of the Spirit, I believe this book is a great read.