I've read just about everything from the heyday of psychical research, which is the era in which these letters were produced. There were any number of books purporting to be letters from the deceased -- "Letters from Julia," "Letters from a Living Dead Man," etc. Most of them make for rather dull reading today, especially in comparison to the classic works on psychical research, or analyses of the cross-correspondences, or actual transcripts of sittings with the great mediums such as Mrs. Piper. Although I'm very sympathetic to this sort of stuff, I'm afraid that this book does fall at the "rather dull" end of the spectrum. The communicators (a sister who died many years before the book was written, a brother who died more recently) are purportedly communicating through their blind sister via automatic writing. Much of the material concerns family matters, and that which is more cosmic or spiritual in nature is hardly in the same league as Stainton Moses' "Spirit Teachings" or the Silver Birch books. There is really nothing to convince the reader that these are communications from external intelligences, as opposed to products of the living sister's own mind. I certainly wouldn't discourage anyone from buying this as a minor curiosity of the era in which it was produced, and you might find a nugget or two that rings true for you.
This book is a series of letters from siblings that have passed away to their still-living sister and family. It's a fascinating read and I couldn't put it down. Many of the concepts that Harry and Helen describe in their new existence have stuck with me and I find myself wondering how my life today is preparing me to survive in that next world. If what Harry and Helen say is true, we have a lot to look forward to!