Deb in the books just doesn't seem like a real character to me the way Deb in the TV show does. But that's true in general - only Dexter comes across as a fully fledged character. The others are kinda props (like Astor and Cody - they're just bizarre - but in the TV show, much more believable).
The books are a lot breezier and less realistic in their approach (probably inherited from the murder mystery genre, which often has an emotionally distant approach that makes murder into a fun game). But if the characters from the books were transported to the TV series, they would seem very cardboard and lacking in believability because they wouldn't match the more realistic and serious tone of the TV series. You can get a cardboard character to do anything the plot requires without a lot of work, but a very specifically drawn character like TV Deb needs to be written with extreme care to avoid coming off false.
Also, I can't recall if the Dexter in the books has ever broken the Code as blatantly as on TV, where he's killed two people who as far as we know, never committed any crime (creepy photographer and the redneck in the bathroom in this season's premiere) and at least two others where the situation was questionable (the apparently reformed pedophile who never committed murder and the habitual drunk driver from S1 who was criminally negligent but not a deliberate killer).