Alright, it's time for some good old fashioned thread necromancy.
(cracks fingers) I've noticed that there seems to be some confusion left here from like a year or two ago about what exactly this show is about. Well, I'll attempt to make things clear without giving everything away with…
Roz's (spoiler-free!) all-purpose Dexter Info Sheet! What is the TV show "Dexter"? "Dexter" is a television series aired on the Showtime channel based (loosely) on a series of novels by Jeff Lindsay. By "Loosely," I mean that the television series so far has not directly followed the novels: in fact, several major plot points as well as a few names have been modified for the television version.
Plot Overview
The Dexter series follows the happenings in the life of Dexter, a blood spatter analyst for the Miami Police Department and a serial killer on the side. Dexter is plagued (or blessed?) by a lack of real emotions, and constantly has to pretend in order to seem "normal." He's good at what he does - acting for others constantly, murder, and blood pattern analyzing - almost too good. Although a few people occasionally find him creepy, Dexter mostly passes off his lack of emotions as genuine social awkwardness and ignorance: only one man constantly finds him suspicious. Of course, Dexter would never let anyone find out about his secret [laboratory, lol]… Would he?
Dexter: The Books!
There are two Dexter books currently available:
Darkly Dreaming Dexter and Dearly Devoted Dexter. The third, Dexter in the Dark is scheduled for a September 18 (One week away!) release in the U.S. and an October 3 release in the U.K. A fourth novel, Dear Daddy Dexter, is said to be in the works. The first two chapters of Dexter in the Dark are viewable by owners of the Dexter series DVD.
As previously mentioned, the television series has strayed pretty significantly from the original novels (But in an awesome way). If anyone wants elaborations on this, feel free to ask. The primary difference, though, is Dexter's "Dark Passenger." The DP (Hurhurhur) is described as something like a separate entity that controls Dexter that he acquired at a young age - something like a voice in his head that drives him to kill. This is rarely if ever referenced in the television series, but is very greatly emphasized in the novels.
More about the TV series!
The Dexter TV series stars Michael C. Hall as Dexter, and man does he ever do one hell of a job. The television series is amazing, and I've made it a goal to impose it on as many people as possible. The first season spans 12 episodes, and each episode is one hour long. While each one tends to contain one or more episodic aspect such as a situation or story involving a side character or a case that needs to be solved on the side of the main objective, the series does follow one main plot. It is highly encouraged that you watch episodes in order.
The first season, for a large part, followed Darkly Dreaming Dexter. The second season, however, is an original story using the same as well as a few new characters. It doesn't seem to have much if any relationship to the second novel. The first two episodes have leaked on the internet as of mid-July.
Is there any reason not to watch Dexter?

Not that I can think of!
DVD Set
The first season of the Dexter TV series was released in DVD format on August 21, 2007. I haven't looked too much into this other than the packaging because I've been a bit busy lately, but I'll detail what I know about it: Obviously, it features the 12 episodes of the first season.
As previously mentioned, the set contains the first two chapters of Dexter in the Dark. I haven't read them yet, but an insert that came with the DVD set gives a hint of what it's about:
! "As if dodging a gruesome competitor and dealing with the disappearance of his Dark Passenger weren't enough, Dexter also faces one of the most troubling things that could happen to any guy… Planning a wedding."
Another insert informs me that the soundtrack will be released on August 28.
Finally, the third insert features a rather cheerful looking Dexter and informs us that the new season begins on September 30.
The special features include two commentary tracks, as well as a laundry list of other things (including some other Showtime series whoring). I can't really say what most of the special features are because the titles of them as listed on the case don't tell exactly what they are.
SECOND SEASON STARTS SEPTEMBER 30!
ARE YOU EXCITED?

Heck yes!!