The Karin light novels are a decent read to complement the manga, especially if you're a huge fan of the series.
I initially had a few quirks with the Tokyopop translation in Volume 1, which seemed to "Americanize" the words used a little much. Thankfully, this seems less evident in newer volumes and it feels a lot more faithful to what the original novel probably had written in it.
As for the little mystery story contained in each volume, they're not exactly what I'd call "mystery" in the sense that you can usually figure out what's going on very easily (though Volume 3 had me guessing a little).
Also, there are new characters added in here and some of them actually reappear in later volumes as well. Some of them seem like they would conflict with the storyline a little (Noel Ambrose, a vampire who sucks morals/ethics), but I think it's done well all the same, especially if Kagesaki approves of the novels herself.
With each volume, the stories seem to get better, so I have high hopes for the later volumes when they come out (the light novels are finished in Japan with Volume 9).
Overall, I'd recommend reading these novels if you like the manga series 'Karin'/'Chibi Vampire', and even if you aren't a fan, it's still worth a read (but reading the manga first will allow you to understand what's going on, because all the novels take place "in-between" whatever happens in the manga.