The Rogue's Diary is looted from several types of junkboxes, along with other rogue items such as and Poison Reagents.
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.