Here is what you have to do:
1. Go to Witcher 2 folder, find there an .xml-file called def_item_books (or something like that).
2. Create a new item (you may just copy an existing book or letter and simply change its name).
3. Go to String editor in REDkit and create strings with following parameters (lets say, our book called "Test_book"):
a) Name: Test_book
Type: String key
Description: Here enter a name of a book, which you'll see in the game
b) Name: TooltipTest_book
Type: String key
Description: Here enter a description of your book (item), which you'll see in the game
c) Name Test_book_entry
Type: String key
Description: Here enter a text of a book (letter) which will be displayed after double click on you book in the inventory
Currently I'm working on a tutorial about creating new recipes\schematics and potions. I'm planning to translate in into English and put it on a new Wiki-page. In case someone is interested in this.