I would think elves and magic-users of all stripes would keep daily diaries. They might include some research notes, material components that need picked up from the local alchemist, theories on why magic works the way it does, and that the new girl apprentice is smoking hot.
While one day's entry could be personal stuff like how they'd love to date the princess, the next day's entry might have magical data like the fact that iron filings mixed with silver filings in a summoning circle help bind demons better.
To make diaries interesting mechanics-wise, when a magic-user or elf finds a mage's diary and reads it, he gains 100 XP based on the magical notes in the book. I would not include any full spells in such a book, just tidbits of stuff that might give a PC mage a "eureka" moment.