One of the things I've been trying to accomplish with my Call of Cthulhu props is a heightened sense of realism for players. Because of the relatively short time-frame of most scenarios both the tabletop and LARP styles of play have inherent problems when it comes to drawing players into the story. Props help alleviate that issue by providing a more immersive experience that helps ground the fantastic events of the story in a real world populated by real people.
That's not something unique to CoC, of course. It's the whole reason there's an entire industry busy providing props for films and theater. One of the masters of the art is Ross MacDonald, who helpfully provides an incredible look at some of the props he's produced at his website. You've probably seen his work before without even realizing it in films like "National Treasure: Book of Secrets", "Van Helsing", and "Zorro".
You'll find some amazing work over there, but I think the "Book of Secrets" and "Frankenstein's Diary" would be of particular interest. The level of detail and artistry is both inspiring and intimidating.