Not really. Its the closest we've got, and the Okudas obviously worked on the production at the time so they had a lot of cred. But a lot
of the entries in the book have got the footnote "conjecture" attached to them. I tend to look at it as only being as valid as any other piece of Star Trek fiction. Certainly an
interpretation of events, but not necessarily the right
interpretation of events.
That having been said, it was an interesting read and an updated edition is long overdue.