https://www.chess.com/forum/search?keyword=Queens+gambit+declined
How you play the Queens gambit declined

Try the exchange variation of the Queens Gambit declined. Don't commit Nf3 and wait to play first Bd3 and then Ne2 so to be able have a pawn break on the center with f3 and a timed e4.
Move order: d4,c4,Nc3,cxd5,Bg5,Bd3,Ne2 e.t.c Find the classic of Botvinnik and Kasparov on this or read the book 1.d4 of Schandorf and QGD of Mathew Sadler it's very informative and written in educative style.

I agree with IM pfren about The Queen Gambit Declined by Matthew Sadler. He has also done the Slav & Semi Slav, which are also good imo.

you have to have to first like chess to understand the queen's gambit declined
it is called declined because black did not accept white's christmas present
Starting Out: Queen's Gambit Declined by Neil McDonald (2006)
https://web.archive.org/web/20140627005627/http://www.chesscafe.com/text/hansen93.pdf
might serve as a general introduction. We have often seen recommendation of the somewhat older book, Queen's Gambit Declined by Matthew Sadler (2000).
https://web.archive.org/web/20140708234438/http://www.chesscafe.com/text/hansen15.txt
A few years ago, there were a number of books with advice for White:
A Strategic Chess Opening Repertoire for White by IM John Watson (2012),
https://web.archive.org/web/20140627105428/http://www.chesscafe.com/text/hansen161.pdf
http://www.gambitbooks.com/pdfs/A_Strategic_Chess_Opening_Repertoire_for_White.pdf
Playing 1.d4: The Queen's Gambit by Lars Schandorff (2012),
https://web.archive.org/web/20140626221508/http://www.chesscafe.com/text/hansen162.pdf
http://www.qualitychess.co.uk/ebooks/Playing1d4QueensGambitexcerpt.pdf
The Kaufman Repertoire for Black & White by GM Larry Kaufman (2012),
https://web.archive.org/web/20140626221508/http://www.chesscafe.com/text/hansen162.pdf
https://www.newinchess.com/media/wysiwyg/product_pdf/955.pdf
A practical repertoire with 1.d4 and 2.c4 Volume 1 by Alexei Kornev,
http://reviews.dailychess.org/a-practical-repertoire-with-1-d4-and-2-c4-the-complete-queens-gambit-volume-1-by-alexei-kornev-chess-stars-2013-304-pages/
https://www.newinchess.com/media/wysiwyg/product_pdf/7288.pdf
and A Cunning Chess Opening Repertoire for White by Graham Burgess (2013).
http://theweekinchess.com/john-watson-reviews/john-watson-book-review-110-repertoires-in-the-age-of-carlsen
http://www.gambitbooks.com/pdfs/A_Cunning_Chess_Opening_Repertoire_for_White.pdf
I think Watson, Schandorff, Kornev, and Kaufman advocated 1 d4 d5 2 c4 e6 3 Nc3 Nf6 4 cxd5 exd5 5 Bg5, while Burgess advocated 1 d4 d5 2 Nf3 Nf6 3 c4 e6 4 Nc3 Be7 5 Bf4.
If the Catalan is your interest, perhaps the most recent book help is to be found in Grandmaster Repertoire 1A - The Catalan by Boris Avrukh
http://www.qualitychess.co.uk/ebooks/GM1A-Catalan-excerpt.pdf
and Grandmaster Repertoire 1B - The Queen's Gambit by Boris Avrukh.
http://www.qualitychess.co.uk/ebooks/1BTheQueensGambit-excerpt.pdf
Not sure what the focus is for the 2016 book, The Queen's Gambit by Damian Lemos.
Advice for Black can be found in Declining the Queen's Gambit by John Cox (2011).
https://web.archive.org/web/20140626233841/http://www.chesscafe.com/text/hansen149.pdf
Recently I got interested in this opening therefore I commonly use the 1d4 opening to get this line but I found a bit of problems in this opening when I reach the midddlegames or sometimes in the opening itself when there are unusual ways they confuse me for those who don't know this is the line(1.d4,d5 2.c4,e6 )this is the queens gambit declined the common line I like is 3.Nf3,Nf6 4.Bg5 or Nc3 to go in the ragozin defence. For those who have a wide experience in this opening can you give me an opinion about this line and some side lines which I can commonly view in this opening