Introduced in 1913, by writer William Hope Hodgson, this 'supernatural detective' has been steadily growing to become one of the more popular literary characters. His primary area of expertise is the realm of Ghosts, though as shown in a great deal of his original stories (of which there were, sadly, only nine by the time his author passed) not all of these were TRUE hauntings... sometimes there was a very grounded/very human cause of some of these 'hauntings'.
What made him stand out from his fellow 'occult investigators' was that he largely had his wits, knowledge, technology and scientific creations to aid him in his endeavors. No telepathy... no psychic powers... no magic. His most memorable device being the 'Electric Pentacle' ( img.photobucket.com/albums/v48… ), a defensive device to keep some spectres at bay (though it's not always effective). Though he has a larger version of this device for larger enemies, (as seen in the story "The Hog") the one he tends to use the most is the normal-sized (and easier to transport) pentacle.
He also has a seemingly encyclopedic knowledge of rituals, incantations, and spells to aid him if need be, the most often used being quotations from the fictional 'Sigsand Manuscript' (think along the lines of the precursor to stuff like Tobin's Spirit Guide and the Necronomicom). This manuscript saves his bacon more than once in the original series of stories, especially the so-called 'Saaamaaa Ritual', a powerful incantation.
Another interesting aspect of the original stories was how most of them were 'being told' to the author and a group of companions that have (apparently) occasionally accompany Carnacki on his investigations.
While it's taken some time for his popularity to increase, (outside of an episode of the 1970's British show 'Rivals of Sherlock Holmes' with Carnacki being played by Donald Pleasance in an adaptation of the classic tale 'The Horse of the Invisible') since at least the 90's he's become quite the popular/influential character in the realm of horror and paranormal fiction. So much so that he's been brought back for newer stories by more contemporary authors.
First was 1992's "No. 472 Cheyne Walk: Carnacki, the Untold Stories" by A.F. Kidd and Rick Kennett, an anthology book (a VERY hard to find anthology book) which largely had stories based on the many off-hand referrences to past (yet unwritten) cases that Carnacki and his collection of friends mention in the course of the original stories (much like various examples in the Sherlock Holmes stories: Giant Rat of Sumatra, Matilda Briggs, etc. etc.). There's a church with an ancient haunting/curse that's just resurfaced in a very dramatic way, the incident involving the unfortunate man known as Aster (who didn't take Carnacki's protections seriously), a most peculiar (and perhaps even dangerous) mansion inheritance, and even a tale spun by one of Carnacki's friends. These twelve stories add more and more to the Carnacki mythos.
Then there was 2010's "Carnacki: Heaven and Hell" by William Meikle, ten more stories dealing with Carnacki investigating more supposed hauntings. With illustrations by Wayne Miller. The tales include a malevolent (seemingly Zulu-born) spirit that's got bloody intentions towards an old British Colonel, a peculiar hill (or barrow) that's seemingly driving a small platoon of soldiers (using the field as a training ground) insane, and other spectral terrors that only Carnacki can seemingly get to the bottom of.
And now there's the very recent, brand new "Carnacki: The New Adventures". Featuring about 11 new stories (and ONE play script) about everyone's favorite occult investigator.
But the ever-increasing world/popularity of Thomas Carnacki doesn't end there.
The comic series 'Gravel' by Warren Ellis is based on a character (a combat magician) named William Gravel. The first story deals with his interactions with a group of modern day occult detectives (and fellow magicians) that he was kicked out of. To get his desired revenge, he has to collect each member's piece of the legendary 'Sigsand Manuscript'. Carnacki is mentioned, and is largely the focus of a portion (third issue) of the story which is an extended homage to the story of the "Whistling Room" from the original short stories by Hodgson.
The 'Ghost-Finder' has also had a good handful of Crossovers:
'Doctor Who: Foreign Devils' by Andrew Cartmel deals with the ubiquitous Doctor (in his second incarnation) teaming up with Carnacki in 1900 to investigate strange murders in a house that has possible connections to a mysterious relic taken from Canton.
The 'Diogenes Club' series by Kim Newman, appearing both in a stand alone series and in his huge 'Anno Dracula' series. In this universe it's a secret British organization that investigates the paranormal. The main character of a majority of the stories is Richard Jeperson, who replaced Carnacki when he retired. There's also mention made that the 'Ghost-Finder' did some investigations alongside Sherlock Holmes.
Carancki himself also joined in on the fun of Alan Moore's legendary series 'League of Extraordinary Gentlemen', becoming a member of the League itself (the 1910-era roster) in "League of Extraordinary Gentlemen: Black Dossier" and "League of Extraordinary Gentlemen, vol 3: Century". It is through Carnacki, (and his harrowing experiences while apart of the League) that eventually reveals to the group the 'black cabal' (led by an Aleister Crowley analogue) that is trying to bring about the end of the world. Unfortunately said experiences lead him to retiring from active duty.
'Sherlock Holmes: The Breath of God' by Guy Adams that not only brings together Carnacki with the Master Detective, but also brings in fellow paranormal investigator Dr. John Silence, future occultist sorcerer Julian Karswell from "Casting of the Runes" (and it's movie adaptation 'Curse of the Demon') and professional oddball Aleister Crowley to try and solve a series of bizarre, supposedly paranormal, murders that are happening across the UK.
It is also through the 'Breath of God' that I got properly introduced to Carnacki for the first time. He's pretty darn cool in this story.
'Sherlock Holmes: The Shadow of Reichenbach Falls', a younger (pre-Ghost Hunting) Carnacki finds himself caught up in the events following the 'Final Problem', and suddenly must protect an amnesiac Holmes from the wrath of Moriarty. Things take a supernatural turn that seems to lead the young Carnacki down his path of tracking the supernatural... the paranormal.
Further Sherlock crossovers (that also act as prequels to the original Carnacki stories) occur in "The Adventure of the Antiquarian's Niece" (featured in Shadows Over Baker Street) which dabbles in the Cthulhu Mythos, and the two cross paths yet again when both are summoned (more or less) to an Abbey out in Grantchester to investigate a case surrounding a very familiar manscript (to Carnacki fans) in "Grantchester Grimoire" (which features in Gaslight Grimoire).
There's also Tales of the Shadowmen, the wickedly-fun anthology series from Black Coat Press that brings together various pulp and literary characters from around the world. Carnacki doesn't appear often... but in Volume 2: Gentlemen of the Night, the Electric Pentacle makes an appearance... proving to be a valuable tool in the 'Werewolf of Rutherford Grange'. Carnacki himself appears in Vols 8 and 10, crossing paths with both the sorceress Palmyre, and the dreaded creation of Guy de Maupassant, the Horla.
I have a feeling we're going to see more and more of Carnacki in the coming years.