You have to be careful with repertoire books as some of them recommend the 6. ...e6 set-up, while others the 6. ...e5 set-up.

King's book mentioned by jaburr is a good option re understanding (it mostly follows the ...e5 set-up). Against 5. Bg5 it follows the Classical approach: 6. Bg5 e6 7. f4 Be7 as mentioned above.

I would like to add 'Mastering the Najdorf' but it is some kind of a black repertoire suggestion.

There's a ton of them (& they age quickly), but here are some of the ones I've read:

Nunn, 'The Najdorf for the Tournament Player' (1988?) -- I started off with this one, was way in over my head, but I couldn't help myself.  It was later split into 2 volumes (one *just* on 6.Bg5 & one on everything else, co-authored w Joe Gallagher).  Can be like reading the phonebook, but strangely addictive all the same.

King, 'Winning w the Najdorf' -- lots of games & accessible explanations.

Emms, 'Play the Najdorf: Scheveningen Style' -- a very useful book for the ...e6 variations (which I've always had more luck w than the traditional ...e5 lines)

Palliser, 'Starting Out: the Sicilian Najdorf' -- I really enjoyed this one, not just for the back-to-basics details, but for the surprisingly substantial content.  

There's a ton more of course -- I don't know much about them though.

http://www.jeremysilman.com/book_reviews_jw/jw_easy_gd_najdorf.html

http://www.jeremysilman.com/book_reviews_jw/jw_Two_Bks_On_Sicilian_English_Attac...

http://www.gambitbooks.com/books/najdorf.html

Hope that's a start.