What if the magic-user was not able to read or understand the new spell due to lack of intelligence or experience levels? To place this type of spell into his spell book, the magic-user must use the first-level spell, write (1st Edition Players Handbook, page 69). Most magic-users use the write spell to record unknowable spells into their spell books, saving these spells for later investigation when the magic-users increase in levels or intelligence. Details on the process of using a write spell are given with the spell itself.
On page 45 of the 1st Edition DMG, the cost of the bottle of ink for the write spell is listed as 200-500 gp; this ink is sufficient to inscribe 2-4 spells. An optional rule here is to make the cost of writing down an unknown spell (including ink and all) 100 gp per level of the spell to be entered into the spell book. This makes the write spell compatible with the section on spell books in Unearthed Arcana (page 79).
Dragon Magazine 147, p34-35