Excellent work by R Talsorian on this one; it's been years since I read through a sourcebook this fast. So, basically what you get here, game mechanics speaking, is a lot more monsters (including a couple categories at the end of higher intelligence monsters you could even play as characters with their lifepaths), some monster components (for the alchemy side of things), an investigation system which I haven't had a chance to try out yet, but looks pretty good, and I do like how they made it so a wide variety of skills can help with the detective work so characters of almost any build could probably find ways to be helpful; and some sample contracts at the end that you could use as player handouts or just give you a pool of ideas to help you make your own.
Where this product really stands out, though, is the "fluff." The book is presented as information gleaned from a discovered journal of a former Grandmaster Witcher. Erland is an interesting character, definitely not just some numbers filed off Gryphon School version of Geralt; I could read an entire book just about him and his adventures. The book begins with Erland's background story in his own words (and gives you an interesting look at the earliest days of the Witchers), and he provides sidebars of his own thoughts on various monsters throughout. Between each category of creature (Necrophages, Relicts, etc) you also get a short story of one of his hunts (and he's definitely Gryphon School so you'll be seeing lots of Signs usage). Even if you aren't into PnP RPGs, maybe you're just a fan of the Witcher universe, this is a highly-recommended book.