I've always dismissed the Gunter book as more of a fun side-adventure than something which may advance the case. It's hard to credit Cooper, Clara, and the story as being genuine or accurate.
For me Jude's (DanCooperHimself ) Youtube analysis on Gunther was insightful and worth my time. This video was the first time I'd encountered Gunther's literary synopsis/notes via - https://dbcooperhijack.com/files/
Although, reading the literary synopsis/notes has not changed my thinking on the Gunther text. I'm a non-believer because... Is it relevant if Cooper and Clara were real or not?
If they were real, I still would question the authenticity of the text. If they were real then why would they not muddled their story with half-truths to avoid being identified. If they were hoaxers then there is no truth to the text.
I believe Gunther's sources at-best could have provided half-truths. I think there is no validation of Gunther's work without identifying the real Cooper. If the real Cooper were identified, I think Gunther's text would then only ever rise to a level of "plausible half-truth". Ultimately, I do not think Gunther's story will prove detailed or accurate enough to "definitively align" with whomever the real Cooper was.
Believing the sources are half-truth's at best, I don't see how the book can be used as a lead to get within proximity of the real Cooper.
Summation - The book will not take us to Cooper. The real Cooper may surprise us and validate aspects of the book as true. However, the sources, if real, likely muddled the truth and Gunther's accounting of "a story as heard" will likely not raise the work above the level of a "plausible half-truth".