The Book of Veils
"If you want some more of those Nick, I still have that cypher I decoded." -- William Hollis, generously offering to increase Nick's Cthulhu Mythos skill

In the campaign, the book being sought after by the mysterious cult that uses an upside down ankh as it's symbol.  The players so far have only found a very incomplete and poorly done translation of the book.

Stuff  characters should not know (highlight the text with your mouse over the text to read it if you're interested):

The propper book is extremely large and it looks more like a copy of the Oxford English Dictionary than a regular mythos tome.  The book is an attempt to catalog Nyarlathotep's forms and it's obvious that the more that were cataloged the more insane the writer became.  The book was written in Russia around 1750 by an unknown author.  It was printed, but surviving copies are extremely rare since most of them have been found and kept by groups that use them in an unintended method.  Sanity loss for reading this tome is 1d8/2d8 and the reader gains +15 Cthulhu Mythos.  It has a spell multiplier of 4x, and at least a dozen different versions of Call/Contact Nyarlathotep are found in it's pages (for different forms naturally).

A few options for cruel and/or fun loving keepers.  The book could contain information on well over a thousand different forms of Nyarlathotep and even some handwritten notes and lists in the pages that run down even more forms that some unidentified previous reader was angry that the original author "missed".  Does Nyarlathotep have more than a thousand forms and his title is a result of the weakness of ancient languages when it came to large numbers, or are some of these false?  Also, there could be some very detailed images of Nyarlathotep illustrating the book which could cause SAN loss similar to seeing the god in that form (probably half the roll rounded down).  Finally, don't discount much of the information in the book being misleading or flat out wrong; Nyarlathotep isn't the crawling chaos for nothing.

Rather than give the players access to that damaging tome directly, I provided an coded version of a bad extremely partial translation to them.  In this form, the book is about half a dozen loose leaf pages.  They're all handwritten and occasionally in the margin someone's done a bit of math.  If the keeper wants to specify what kind of code was used to encrypt the pages, I recommend a palandromic one.  Whoever encoded it took the place of the letter, reversed the order of the numbers and continued doing that until he got a palandrome (a number that's the same forward and backward).  Then he divided the palandrome by 26 to determine what offset the number would have.  It's not hard to decode when you understand it, but it would require someone sitting down with a pencil for a few days doing the math needed to decode the text.  In this form there are no spells included (it would probably be dangerously incomplete if the keeper chose to have the text be a spell).  Sanity loss is minor; 1d3/1d6, and the reader only gains +2 Cthulhu Mythos.