Farming Pages/Tomes of Crafting
Do I have to be on a certain difficulty or Act? Do they only drop from Champion/Elite mobs? Are they particularly rare?
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You will start finding them once you being exploring Nightmare difficulty where they are uncommon but not particularly difficult to obtain. The Tome of Blacksmithing and Tome of Secrets become available in Hell and Inferno respectively.
A few reports of seeing pages in Normal difficulty seem to be leftovers from the beta period or unsubstantiated.