What's the meaning of "validate the voice of this book" in the following context?
I thank Lisa who reached out to her vast network to help validate the voice of this book.
Writers speak of "finding a voice" when they write. This is especially true of fiction, when writers are not necessarily speaking as themselves. They want their work to have a consistent and identifiable narrative. The reader should be able to either identify with "the voice" of the narrator, or otherwise picture the narrator as a particular kind of person. But even a factual book has "a voice", as it may express a particular opinion, for example a political opinion.
I believe the writer is thanking one of their proofreaders who evidently checked the facts that backed up what the writer was trying to say.
You don't say whether this credit is found in a work of fact or fiction, but let's say for example that a novelist created the character of a doctor, but the author is not medically trained themselves. They may reach out to someone with medical knowledge to check the facts and "validate" or add credence to the character by ensuring what is written is correct.