Rather than commit their wizardly knowledge to spellbooks, the ancient giants fashioned necklaces of the phantom library out of Siberys dragonshards. Whenever they needed to prepare spells, they donned a necklace and studied the glowing runes that surrounded them.
Lore: The ancient giants were powerful wizards who used magic necklaces to store their spells (Knowledge [arcana] DC 15). A necklace of the phantom library functions exactly like a spellbook, except that scribing new spells into it takes less time (Knowledge [arcana] DC 25).
Description: A necklace of the phantom library is a necklace made of golden chain with five lozenge-shaped Siberys dragonshards set among the links at equal intervals.
Once you don the necklace, yellow-orange sigils appear in the air about two feet from your face. These sigils are the spells inscribed within the necklace, and you can move from spell to spell with a thought, the sigils spinning through your field of vision (an effect that looks a little like a scrolling microfiche reader).
Prerequisite: Anyone can don a necklace of the phantom library, but the spells within it are useful only to wizards.
Activation: It takes a standard action to put a necklace on or take it off. Moving from spell to spell is a free action. As a practical matter, you’ll wear the necklace only when you’re preparing your spells, so the exact timing of your actions doesn’t matter.
Effect: A necklace of the phantom library is primarily a storage device—the giants’ equivalent to a present-day wizard’s spellbook. A necklace holds 500 pages worth of spells, and you can purge the necklace of unwanted spells with a moment’s thought.
You can inscribe new spells into a necklace of the phantom library by wearing the necklace, then tracing the necessary sigils in the air before you. You can thus copy spells without paying the 100 gp per page material cost, and the process takes only 8 hours per spell.
Whenever you don the necklace, the glowing symbols appear in front of your face. They obscure the rest of your vision, granting total concealment to anyone who isn’t adjacent to you. Anyone else can see you just fine, however, making it a good idea to take the necklace off prior to combat.
Aura/Caster Level: Moderate illusion. CL 11th.
Construction: Craft Wondrous Item, programmed image, 7,500 gp, 600 XP, 15 days.
Price: 15,000 gp.
Source: Explorer's Handbook