There are a total of 324 skill books in Fallout 3. With this perk, you can potentially increase the effectiveness of all 324 skill books by a factor of two, changing the potential skills increase of 324 to 648 skill points when all books are read. This, combined with the fact that two types of skill books respawn, can mean a potentially infinite number of skill points available. You cannot raise any skill above 100.
“With the Comprehension perk, you gain double the bonus from reading magazines and one additional point from skill books.”— In-game description
You gain an additional skill point for reading a book (for a total of 4 instead of 3), and double the temporary skill points from magazines (for a total of 20 instead of 10). There are 53 skill books in New Vegas; 65 if you have Dead Money and up to 73 with Honest Hearts, if you are patient enough to reload many saves outside caves with a Workbench crate. In Old World Blues there is a crafting recipe for 1 additional skill book per skill (13), and two world placed sneak books, and 1 science book. In Lonesome Road there is a skill book for each skill, bringing the total to 102. As in all other instances of the Comprehension perk, it will not affect books and magazines read prior to selecting this perk.