Note: Text is paraphrased from Call of Cthulhu 4th Edition, by Sandy Petersen. 

Q: Why does it take such a long time to read a grimoire?

A: 

(1) These thick books are scribed by lunatics, are often hand-written in terrible
scripts, and use archaic language. Large portions of the manuscript might
not even be in the Roman alphabet - instead occult cyphers are used instead. 
The books are written for adepts, not laymen, and terms are used without
explanation. 

There is no index. 

There is no glossary. 

There is no table of contents. 

There might not be chapters, paragraphs, punctuation, or even spaces between
words. Different volumes in a series might be written centuries after each other
and different phraseology might be used in each one. Each new volume in the 
series requires starting one's study anew. 

(2) Skimming through a book doesn't offer much - just SAN loss. A reader 
has to read the book slowly. Not only must every word be studied, but 
compared with other books related to the subject, and not just occult
books either. 

(3) The books are ancient and difficult to read. Many of them are ancient
and must be treated with care. A reader must wear gloves and turn 
the pages with forceps.

(4) Again, it bears repeating that these books are huge. Even the Golden
Bough is 13 thick volumes long. 

(5) As any student will tell you, one may only study so long in a day before
becoming tired and bored. These books are poorly-written, unclear, and
long. A bored reader won't make the conclusions necessary to understand
the work. 

======================
  Study Time For Books
======================
Note: This was lifted from The Unspeakable Oath, issue 6. 

There are many Mythos tomes available for Investigators to study. Some specify
a certain number of weeks or months to study it. In the opinion of this Keeper, 
that simplicity hurts the game. This system, while involved, is fair to players
and Keepers. 

* Determine when the book was written or translated. 

* Compare it to the year of the campaign. Determine how old the book is. 

* If the book was written on or after 1000 AD, divide its age by 100. 

* If the book was written between 1AD and 1000 AD, divide its age by 500,
   then add 10 to the result. 

* If the book was wirrten before 1AD, divide its age by 1000, then add 15 to
   the result. 

* The result is the Age Code. 

* Add together the Age Code of the book, the maximum Sanity Points it costs to read,
  and the Mythos points it grants. Multiply that number by the Spell Multiplier. That's
  the number of hours it takes to read. 

===============================
    Making Progress on the Books 
===============================
   The progress an Investigator makes with a book depends on where he studies the book. 
He will make more progress at a large university or city library than he will on the road. 

An investigator who uses a library or university first makes a Library Use roll and then 
 Other Language (if needed). If both rolls succeed, he makes progress. If he has no access
to a library, then Other Language (if needed) is enough. If the book is in an archaic version of
his own language, then he will need to make an Own Language roll!

 Study Place					Multiplier
Large University, Huge Library			x 1
Medium-Sized University, Medium Library		X 0.5
Small University, Small or Private Library	x .1
On the Road or No University/Library		x .05

Use the above as a guideline - although Miskatonic University is a Small University, doing Mythos 
research there is x1, not x.1 . 
Let's say Tatsumi-sensei studies the G'harne Fragments at Tokyo University (a Large University, x1) and makes both 
his Library Use and Other Language (English) roll. He spends five hours studying the book (5 x 1 = 5)
and deducts five hours from the time remaining to finish the book. If he read the book at home, 
he'd multiply five hours x .05 and remove 25 minutes from the time remaining to finish the work.

Five hours of studying a day is about as much as one can do. If the player wants to study six hours, he must make a POW x5
roll along with his other rolls. Seven hours requires a POW x4 roll. Eight hours requires a POW x 3 roll. Failure means the Investigator
has exhausted himself and completely negates the day's progress! 

This system gives researchers something more to do while the social investigators run about the town. The researchers probably
have to find libraries to "get their Rupert Giles on" while everyone else is busy. This also gives the Keeper the chance to have
the Investigator tailed or attacked. It gives the game more flavor.