It's easy for theology wonks to critique this book for not engaging all the thorny issues, but that's not its intention. I have some problems with his conclusions, but this really is a helpful book to give Old Aunt Gladys at church.
He notes how the persons of the Trinity *subsist* in the essence. He's drawing upon Boethius' work and I think that is problematic, but it is a good start, at least. He follows the standard line in apologetics by noting how all the persons are divine, yet there is one God; yet each person is not the other. Etc., etc. Again, this formulation, if taken too seriously, will lead to problems down the road (e.g., relations of opposition stuff); however, it really is a good way to get the conversation going.
Conclusion:
This is probably the best take on the Trinity as it relates to "Sunday School" teaching.