Oh, there seem to be a million characters I would love to cosplay from books. Off the top of my head...
Cara, Kahlan, Nathan, Richard in his War Wizard outfit. All from Terry Goodkind's Sword of Truth Series. *madly lusts for the last book, withers away before November*
War or Crowley from "Good Omens".
Admiral Thrawn from the Star Wars books (Timothy Zahn)
R. Danieel Olivaw or Elijah Bailey from the Robots series (Caves of Steel, ect) by Isaac Asimov.
One of the Harpers or Dragonriders from the Pern series.
Pretty much any character from The Tales of the Otori series.
Doing something from "Mrs. Frisby and the Rats of NiMH" would be awesome, as well as the book "A Wrinkle in Time"
Going in one of the red dresses from "A Handmaid's Tale" or doing a cosplay from "The Last Unicorn" (its been forever since I read some of these books, I can't remember names right now ^_^;; )
Eve Dallas or her partner (can't remember her name right now) from J.D. Robbs "in Death" series
Or going Arthurian legend style with a character from "The Mists of Avalon". It would be hard to narrow it down, character-wise in that book.
I think that's a good start? ^_^