A Piece of Copper

This is part of a little secret in the dungeon journal. The entire sequence takes five weeks to complete.

Each week (after reset), open your Adventure Guide (Shift+J), select the "Suggested Content" tab, and in the left pane scroll all the way to the right. You'll get one of these quests, in order:

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• Week 1 - A Piece of Copper - Rewards 1c and the achievement Lucky Penny.
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• Week 2 - A Piece of Silver - Rewards 1s
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• Week 3 - A Piece of Gold - Rewards 1g
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• Week 4 - A Bag of Gold - Rewards 10g
  
• Week 5 - A Curious Coin - Rewards [item=202107], the achievement Curious Coin, and a message from our mysterious benefactor.
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The final message can be found on Warcraft Wiki: https://warcraft.wiki.gg/wiki/A_Curious_Coin

so somebody reverse pickpocketed me to put money inside my book?

1) Open Adventure Guide (shift-j or the skull on your micro menu)
2) Click on the second tab on the bottom called "Suggested Content"
3) Left pane of that frame has arrows in lower corners - hit right arrow
4) Continue hitting right arrow until the end
5) First time, last panel reads "Copper Coin," click "Sweet Free Money!"
6) Complete the quest
7) Each week the next entry in the series will appear in the last panel
8) Repeat until questline is complete (ends at week 5's "Curious Coin")