As you can see above, the design archive cover features a sketch of the GX main visual, however if you ordered the book through Satelight's online store you got a bonus: a slip-case that features alternate, unused art. This particular image was previously featured in Fujimoto Satoru's comiket staff book, along with other images that he drafted to be used as GX's key visual. Personally, I prefer this image to the one they actually used so I'm excited to see it featured here as more than a rough sketch.
Anyway, getting to the actual content of the book here's a look at the table of contents. Just like the previous books it includes all of the magazine art that was used as promotion during GX, as well as all the CD covers. From there it moves onto the production art sheets for each character.
Perhaps the biggest draw of the design book is getting to see the original designs. They are often quite different from the final products in various ways. You also get a cleaner view at all angles of the designs than you do by watching the series. A good example of this is the "inner" suit designs for all the characters, which we only briefly saw in the last episode of GX when they shed their armor.
Sometimes we can learn about some early aspects of the series that were changed during later in production. For example, the design page for Micha's "Burning Heart Mechanics" actually is labelled as "Ribbon Removed Version," perhaps implying that she was powered up by simply removing the ribbons that give her twintails. The unique part of this is that she retains the rest of her clothes, where as in the final design they all burnt up.
There are also design sheets that detail how the character's gears and weapons work. Sometimes it offers us a look at techniques or fighting moves that were abandoned or unused during production. This doesn't just apply to the girls, but to the Noise as well. In one page dedicated to the Banana Noise we can see it doing some moves never seen during the series.
During the series, sometimes you'll notice little details on characters that have been fully expanded upon in the design book. For example, did you notice that in episode 2 of GX Itaba had a small "Mantis" plushie from her favorite show: Lightning Detective Ban?
Last but not least there are a ton of interviews in the back. Interviews with the creators, the director, the character design team, the background design team and so on. Of course theses are all written in Japanese and might not be so exciting unless you can read them. I will give a brief overview of the interesting bits here, but someday in the future I hope to be able to offer full translations of all of them.
In the interview with Kaneko there was one part that stood out to me. When they asked him "if there will be more after this, what new things would you like to challenge?" he responded by saying that he would like to create an animation (he doesn't specify if it would be a movie, OVA or full series) that takes place prior to season 1. It focuses on the exploits of Tsubasa and Kanade as they get involved in battle operations on Okinawa (as opposed to the traditional setting of Tokyo). In the event that he can't fulfill that dream, he hopes to write a story that focuses more on Genjuro, Miku and the 3 Girls (Ando, Terashima and Itaba). However he says that plans like those often tend to fall apart when he puts them together, so perhaps don't get your hopes too high. Still, it's an interesting look into his personal wishes for the future of the series.
All in all, the Design Archive is well worth the price you pay for it. Books like these usually tend to sell out within the first month or two, so if you're interested I highly recommend picking it up as soon as you can. The next book I'm expecting to be released is the Symphogear GX visual collection from Megami Magazine. It hasn't been formally announced yet, but once all the BD volumes are out and the concert is over they should have all the content they need to fill a new book. It will probably come with another exclusive poster so keep your eye on the blog or my twitter for any information when it becomes available.