I came across an interesting problem the other day with my investigators. One is sitting in the hospital with nothing to do, so he told me he was gonna start writing down all he knew about the Cthulhu Mythos, to make a tome to pass on to other fellow investigators. I thought this over and came up with a few ideas. 

(1) Cthulhu Mythos - There should be some sort of limitation on the amount of Cthulhu Mythos skill one can put into a tome they are writing. I chose EDU for the max amount of Cthulhu Mythos one can put in. To write the book and impart the knowledge I had the investigatorroll a successfull Cthulhu Mythos.

(2) Other Skills - In the _Keeper's Compendium_ more than just Mythos and spells are in books. If you read a book, you could get a skillcheck for some skills, like Occult or Astronomy. I allowed the investigator these too if he made a successfull skill roll for the skills he wanted to impart into the book.

(3) SAN - Sanity loss is an important part of every mythos tome. The best idea I had for this was comparing the total amount of Mythos skill in a book to the rulebook's section on mythos tomes. If I could match up a book with the same amount of mythos skill gained, I used that amount of SAN loss.

(4) Spells - Another important part of a mythos tome is its spells. I simply allowed the investigator to put any spells he knew in the tome.

(5) Amount of time to write- Mythos tomes are often quite long. I suggest that for every +2 Cthulhu Mythos Skill or Spell in the book it takes 1d6 months to write. For example: A tome with +6 Cthulhu Mythos and 3 spells takes 6d6 months to write.

(6) Amount of time to read - I just used the same amount of time it took to write. See above.

In conclusion, I think this is a fairly good way for an investigator to write a mythos tome. I'm interested on any suggestions to the system, so feel free to post them here.