Books improve hero health and magic attack. They are complex items, so sometimes, their crafting may be confusing. Here is a guide that may help clarify the matter.
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To craft a Book of Tales, you need one Flaming Heart and two Pastor's Seals.
Note
Each of these items consists of 20 fragments that you get in Campaign missions. Thus, you need 20 Flaming Heart fragments to craft one Flaming Heart item.
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To craft a Book of Prophecies, you need a Book of Prophecies Recipe and two Books of Tales (i.e., two Flaming Hearts or 40 Flaming Heart fragments and four Pastor's Seals or 80 Pastor's Seals fragments):
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To craft a Book of Fate, you need a Book of Fate Recipe and two Books of Prophecies (i.e., four Books of Tales or, in other words, four Flaming Hearts or 80 Flaming Heart fragments and eight Pastor's Seals or 160 Pastor's Seals fragments):
Note
Several heroes need these items for upgrading. Be careful when equipping an item and double-check if you are upgrading the desired hero.