Rogue's Diary

Source
Item known to be contained in any of several types of Junkbox, along with other Rogue items such as Flash Powder and Poison Reagents.

Symbol
While it's possible that a rogue would use invisible ink for such a diary... it's also possible (and more likely) that this is a symbol. This book is suggesting more that a rogue needs not a diary, but has a good enough memory to remember everything. A rogue would be smart enough to know that once information is written on paper, the enemy could get their hands on it. So while this diary used to belong to a rogue, it symbolizes what the intelligence and wits a rogue truly has.


 * Note: Many other grey items that are found across Northrend that are usually discarded (very much like this item) usually have a symbol or deeper meaning behind them, like this one.