Headhunter, Ghoul, and Cutthroat are all fine books, but they've got nothing on Ripper. If you've only got time for one Special X novel, I recommend you make it this one, as it stands alone the best.
There are, of course, references to previous Slade outings here. The Mounties discuss prior cases, mainly Cutthroat, although the other two get their nods too. There are some minor callbacks you won't get, a returning minor character you won't realize is a returning minor character, and the impact of certain events will be blunted, if you've not read the previous books. However, the overall story stands alone and does so very well despite bringing together the protagonists of the earlier stories for our first full Special X outing. Despite juggling three major story threads, Slade keeps the pace churning as scenes swap between DeClercq's lower-key role as the coordinator of his team, Nick Craven's work gathering evidence and interviewing witnesses, and Zinc Chandler's attempt to prove he's still got what it takes to be a cop after what went down on his trip to Hong Kong five years ago (chronicled in Cutthroat).
Then, of course, there's the fourth POV we've all come to see: that of the killer(s).
Slade's no stranger to spilt blood and grue as every reader should know, but the victims in Ripper are dispatched in some truly heinous ways. From having one's head ritually skinned and scalped, to being on the receiving end of face-melting acid; from having one's intestines removed via brute suction, to having one's head pulped in a vise one crank-turn at a time, if you're looking to compile a list of "worst ways to expire", Ripper will easily provide a dozen entries.
Ripper also throws a ton of history the reader's way, Slade's trademark calling card. Police procedural stuff is lighter in this book than the earlier novels, but you'll get more than your fair share of background on everything from Jack the Ripper and Aleister Crowley, 'locked room' mysteries and the tarot, John Dickson Carr and Ellery Queen. Slade's books always end with a bibliography, just in case the info dumps don't tell you everything you wanted to know about a given subject raised by one or more characters. Ever the cheeky one, Slade even has one of his characters remark about Chief Inspector DeClercq, "He thinks novels should have bibliographies so you know the author's done his homework."
All that's well and good for a solid four-star read, but what earns my fifth star here is Zinc Chandler's part of the story. Since he's been out of action for the last five years after suffering serious head trauma in Cutthroat, Chandler's been looking for an 'in' that will let him prove (both to the RCMP and himself) that he has what it takes to still be an effective cop. DeClercq, sensing a way to ease him back into the action, recruits him for a safe but cerebral assignment: he's to play Inspector against a dozen hardcore mystery authors who are staging their own live-action "Catch A Killer" scenario for the chance to earn a $50,000 prize. Chandler's role is to see if his talent as a cop makes him better equipped to solve a mystery than those who create them for a living, which is a fantastic idea for a mystery in and of itself.
What should be a quiet, simple weekend getaway of fun turns into Slade's own homage to Christie's classic, And Then There Were None as the guests discover the mansion has been painstakingly fitted with all manner of vicious, violent, and horrible death traps. The planes which flew them to the island have departed, the power is out, the phone lines have been cut, and a massive brewing storm ensures there's no possibility of a rescue party. As bodies pile up, Chandler comes to the cold realization the killer has to be among them--the only question is whether or not he can solve the mystery before the party of fourteen is whittled down to a party of none.
This sequence alone is worth the price of admission: Slade obliterates a smorgasbord of secondary characters in gleefully sadistic ways, all the while taunting both Chandler and the reader with a never-ending series of conundrums and riddles. How do you fire a crossbow without your finger on the trigger? How do you cut a man's throat in the middle of an empty room? Read for yourself--I do spoiler-free book reviews.
The takeaway from all this is Ripper's a gas from start to finish. Slade's love/hate letter to the finest mystery scribes of all time is blanched deep red with his penchant for bloodletting. One of the scribes assembled on the island for the game explains he writes horror-whodunnits, a very niche market: the bloodier bits turn off the traditional mystery fans, while the mystery aspects don't do much to whet the typical horror fan's appetite. The man was referring to his own publications, but he just as easily could have been Slade, sneaking in a quick fourth-wall breaker about his own writing. No matter--Ripper rocks, and remains my favorite Slade offering to this day.