@ Schattenjäger
"The Gabriel Knight series: Mystery/Horror adventures with historical backgrounds and quality storytelling."
As Sceptic said the Broken Sword games could be considered similar, but they don't have the same mood and coolness. George is not Gabriel Knight, period. And Nico is cool and all, but she's not Grace. The mysteries are not half as interesting. And all that globetrotting actually hurt the games.
So for the closest thing to Gabriel Knight you should wait a couple of months for
Gray Matter, Jane Jensen new game. Though she has been making it since before i started playing videogames, so if you guys joke about Age of Decadence being vapourware...
A game i discovered quite recently and i think can be seen in that way, though it's not third person and it's more of a mixture of Myst like puzzles with a dungeon crawler, though with the dungeon crawling limited to traps and exploration and having no real combat, is Azrael's Tear. Dark mood and lots of atmosphere? Yup. A whole lot of historical background? Yay. Very deep and cool characters? Totally. The storytelling is quite amazing too since the story is quite simple at first (you are a faceless high tech tomb raider having to enter a massive underground complex to find the grail) and slowly start to discover the story of each of the characters you come across by means of reading their journals, interpreting small details in the complex's rooms, reading the manual (that has a lot of background information and the like), and asking the right questions during the short interviews you have with them, limited to between one to three questions each and impossible to repeat other than by reloading. And almost everyone is totally insane after a couple of centuries keeping guard down there, and almost everyone is lying to you or otherwise trying to manipulate you in one way or another, up to the most cool twist ending ever, which is kind of awesome since videogames are usually inhabited by really honest conspirators and the like.
Another one could be Black Dahlia, if you are into Nazi Occultism and very noir games. But on that i can't really comment, a friend recomended it to me and told me about it in ways made it sound like what you are after but i haven't played it yet myself. Most reviews, though, mention puzzles being crazy dificult to solve without a walkthrough.
