So, this is the umpteenth book in his series, you either know the series already or you've read the promotional description and/or other reviews.
This isn't an ongoing series, you can pick it up anywhere -- and in fact, this particular entry ends out with two previously published short stories from the first and also the previous release, somewhat-literally spanning his corpus so far. They're fine selections, and are representative of the generally high quality of his work from the beginning up to now (at least in his short stories); so if this is the first entry you read in his series, there you go, you know what to expect from the others, buy with confidence if you liked the stuff here. (Though I kind of recommend skipping his novels.)
Did Rusty invent the stories himself? -- which is a different question from whether they actually happened. The books are listed purely as fiction, and his biography is definitely (and humorously obviously) fictional about the Bigfoot parts. So I read them as good fiction.
This entry, however, starts with something Rusty hasn't done before: overtly insisting that while he can't vouch for all the details, not having been there personally, he does swear he didn't invent any of these stories but heard them from other people (just as he says) whom he then contacted again several times to rehear them and record them on tape -- and that he doesn't ever include anything that he judges to be people just playing around with him.
That could be true; I have no way to tell. Internal evidence indicates quite clearly that these aren't transcripts, so if they're real accounts (in the sense of being stories really told to him by other people) he has polished them up for presentation, including with details that would make no sense from an oral delivery.
Moreover, he claims he sometimes (usually?) changes the locations as well as the names of the people involved, although that's somewhat hard to believe. This volume all happens in or near Yellowstone, and he acknowledges he didn't change locations for these stories (or obscured the locations only a little); but the location characteristics in most (if not all) of his other stories are specific to various importances of the stories, too -- and to the same degree as in this book! Maybe he was told that the incident happens near X-town or on Y-island, and he changes it somewhere else, but then he researches some travelogue detail to flesh out the narrative color of the new setting for verisimilitude?! The result reads great, if so! But that's a lot of work for his art, if so.
Then there are disjoints with the real world on occasion, particularly in those stories which involve large searches (at least one of them in this book), which by their nature should involve surviving press coverage -- but when I've occasionally gone to check, I've found nothing on the internet about searches for missing people in even the general area of the story's setting. Is that because he changed the setting? -- but those settings are usually important to the shape of his stories, too.
On yet another hand, I think a style comparison with his overtly fictional work (his novels), shows a lot of difference: reading The Bigfoot Runes, where he's overtly welding together some striking anecdotes from his earlier books into one plot, I find his basic narrative ability a lot clumsier and unpolished than in his short stories. That might be an indication that he really is just "redacting" (in the technical sense of editing for presentation) other people's story material told to him, in the short stories. Or maybe he's just much better at writing anecdotal short stories than a novel made from events in his anecdotal short stories -- though I find that hard to believe.
In conclusion: I enjoy his short stories a lot, and they've never flagged in quality. They are, at the least, well-researched representatives of a wide variety of Bigfoot encounters. I don't recommend taking them as research data, but they're evocative. And aside from scary tension they're perfectly G-rated, much like the original Legend of Boggy Creek now that I think of it; and, even if they're fictional, rather inspiring, to me anyway. I'm glad to recommend them to anyone who appreciates Bigfoot stories. Rusty, one way or another, is one of the great American authors, not far below the tier of Mark Twain in many regards.