uhhmm have you read pandora hearts? they investigated of cases a little but the story revolves around discovering the identity of the protagonists and the mystery that surrounds the story...you might like it
Pandora Hearts ~ seconding. There are numerous aspects as to why they are similar, which is why my friend and I often compare Kuroshitsuji and PH. Here are some comparisons: -the relationship between Ciel and Seb are almost exactly parallel to Oz and Gil -romantic subplot: CielxWhatshername and OzxAlice -mystery and supernatural/fantasy mix -Ciel and Oz are both rich brats that find character development and are super adorable -Seb and Gil both wear black and are great fighters And so on...
Karneval ~ also very similar! Those three (Kuroshitsuji, PH, Karneval) all go hand in hand... But PH is way better. That's just me though.
Gosick ~ it's solving mysteries and stuff so it's a good match. Seconding.
Death Note ~ lots of mystery cases going on here—mainly mind games. It's very long, but worth the read if you're really into detective stuff coexisting with the supernatural.
Manga aside, a book you should read is "And Then There Were None" by Agatha Christie. If you really liked the part in Kuroshitsuji when they're in the Phantomhive mansion and they have to figure out who the culprit is, you'll love this novel. There's also a part similar in Gosick.
________________ I am vindicated! I am selfish; I am wrong! I am right, I swear I'm right. Swear I knew it all along And I am flawed! But I am cleaning up so well I am seeing in me now The things you swore You saw yourself...