What is the most up-to-date and comprehensive book on the Najdorf?

You got to study games.  I am not sure because my connection was failing at times, but I didn't see Alex Yermolinsky and Robert Hess comment on the Radjabov/Grischuk game.  If they didn't, shame on them.  If they did, then shame on chess.com for not providing better streaming.

Here is the game.  Study it.  You don't need books.  Study one game at a time.  You will improve.

You need neither up to date nor extensive books. When I first started studying openings, I thought: "man I want the biggest most up to date books with the longest and most complicated variations". The truth is all those variations are unnecessary and can bog you down. Bigger is not equal to better.  

1) Works on single chess openings basically started as pamphlets back in the 70s/80s. Basically annotated lists of variations. Then as they got bigger, someone got the marketing idea to turn them into books. Then as always happens with books, people bought the books with the most pages.  This is a notorious problem with literary books, people buying quantity over quality, and I'm pretty sure it's the same or worse with chess books. Why buy a 110 page book when there's a 220 one right there, despite half of the content houdini variations.   

2) Unless you're a GM or big into correspondance, being "up to date" has become nothing but marketing. Back thirty or forty years ago, new variations were coming out left right and centre. What was once a sure win could suddenly turn into a draw, or even loss. What was thought a fine defence might change to one extremely hard to defend. In the computer age, things like that don't really happen anymore. Whether you buy a book from now or ten years ago doesn't matter.   

Check out the following from about a week ago.

While the header is on stuff for Black, the post I put includes both Black repertoire and objective books:

http://www.chess.com/forum/view/chess-openings/best-book-for-najdorf-as-black