I do love the Looking Glass Wars series.
I also like how the series goes beyond the actual trilogy without getting into heavy marketing tie-ins.
First off, there's the soundtrack which is an awesome concept, and an awesome album (but won't be everyone's cup of tea. Listen to the samples before you buy.) Then there are the games. Some are your basic on-line tie-in games that aren't completely unusual for books. But! There's also a role-playing game and a card game. And, there are the spin-off books.
Princess Alyss of Wonderland Frank Beddor
This is Alyss's scrapbook and journal that she kept while trapped in England. Lots of drawings, pictures, and even letters (that come out of the book) and a deck of playing cards. It doesn't really add to the series, but is a fun addition for mega-fans.
Yes, this is way below 100 pages, but I'm mentioning it because I thought I was going to use it for a paper I wrote in grad school but decided not to.
Hatter M: Volume 1 Frank Beddor with Liz Cavalier, art by Ben Templesmith
As far as I can tell, this was originally published as a series of comic books and then gathered and printed in an omnibus edition.
This graphic novel tells the story of Hatter M, while he's searching for Alyss in our world. Due to his awesome weaponry and martial arts skills, it's a great adventure that works really, really well in a graphic novel (full color!), almost better than it would in plain text. The only thing is that I think the graphic novel is aimed at adult readers of the series, even though the series (in the US) is published for middle grade readers.
I don't like talking about art, because I feel like I can't do it intelligently, but Templesmith makes awesome use of color.
AND! In October we finally get the last book in the trilogy, ArchEnemy, and a new volume of the Hatter M graphic novel, Mad With Wonder.
I'm super excited!