Book of Hoeth

The Book of Hoeth is one of Saphery's most treasured artefacts. The very first pages are laid down in the hand of Bel-Korhadris himself, and every subsequent entry bears the seal of the greatest Loremaster of his time. Yet a great many of the Book's pages remain blank: the scholars of the White Tower are aware that even their knowledge has limits, boundaries that must be driven back if a complete mastery of magic is to be achieved.

Source

 * Warhammer Armies: High Elves (8th edition) -- p. 65