Rook endings are the most frequent of all endings and perhaps the most difficult as well. So, I believe studying rook endings would pay off best. Pawn endings are also important ofcourse. Endgame Manual is an excellent book on endings, a great recommendation.
What do you think more important?
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Pawn endgames first, then rook endgames, because some rook endgames transform into pawn endgames, but the opposite doesn't happen 
As for good general books on endgames, you have Averback's, Silman's, Dvoretsky's, Alburt & Krogius' and Müller/Lamprecht's in no particular order. One of them is enough : if you know everything in only one of these books, you'll probably know more than most 2300 rated players
On top of that, an excellent book dedicated to rook endings is John Emms' survival guide.
hicetnunc,
Vous devez etre un joueur d' eches excellent. Votre conseil a Kosmeg est de loin le meilleur que j'ai vu sur ce site.
Thanks for your suggestions. I think I will get Dvoretsky's Endgame manual, but I will think about it a bit more.
Check out this review by Silman...
http://www.jeremysilman.com/book_reviews_js/js_dvrskys_endgm_mnl.html
I am a 13 year old and I want to really improve my endgame skills. I have a FIDE rating of 1678 but it is just my first one so it couldn't be really high. I know that I play for 1850-1950 FIDE elo and I want to have achieved a level of almost 2000 by this time next year. I can tell that the best part of my play is the opening, but I'm quite weak in endgames. I have Silman's complete endgame course and I have read all of it before the expert level. Now I want something more specific and here is my question?
What do you think is more important? Rook endgames or pawn endgames?
please feel free to also reccomend books that you've read about them. I like the challenge so I want the book to be a really difficult one. Not for a 1850-1950 player like me but for a 1900-2050 player or something like that, so that I will find it useful for many many years.
Any suggestions?