Releases for the Lord of the Rings LCG
There are two ways you can go about collecting the expansions, one is order of release, the second is chronological order of events.

Here are a couple good resources for choosing what order to buy the expansions in:

New Player Buying Guide (Updated January 2015)

This guy went through the effort of placing each expansion into chronological order of events according to the books, I've quoted his summary below.

Play Chronology / Quests Timeline


  To Summarize…
  
  The Quests in Chronological Order
  
  Over the Hill and Under Hill (2941 TA) [Gadalf: Sending an Hobbit to
  slay the last fire breathing dragon. What could possibly go wrong? On
  a totally unrelated sidenote, dang these are some good pipeweed!]
  
  Battle at Laketown (2941 TA) [Bard: Stupid Dwarves, stupid fat Hobbit,
  and most of all stupid procrastinating weed addict wizard. In the end
  I did all the work and all saving laketown and all slaying Smaug.]
  
  Shadow of Mirkwood (3009 ~ 3017 TA) [Gandalf: Oh, Sauron is in action
  again? Ah oh… he must be searching for the One Ring….. I am kind of
  suspicious about Bilbo's ring. Maybe I will try and learn more about
  Bilbo's ring by questioning Gollum]
  
  Core Set (Spring/Summer 3009 TA) [Thranduil: Oh shizzle, the fortress
  of Dol Guldur, which White Council cleansed couple decades ago, is
  sending out evil vibes again. I better send warning to all nearby
  allies]
  
  Khazad-Dum (3009 TA Early Winter) [Elrond: I'm hearing a lot of
  reports about evil things stirring in the Misty Mountains and Mirkwood
  and such. But oddly no report came from Moria for… 15 years, that
  seems like a long time. Maybe I'll send someone to check them out, to
  see if they are doing okay.]
  
  Dwarrowdelf (3009 TA Winter) [Elrond: Sweety how was your journey over
  the Mountain? I know it was wintertime but Lorien was just too close
  to Dol Guldur. What? You were attacked by the Orcs? Come to think of
  it, the scouts I've sent to Moria hasn't returned yet. This is
  concerning… maybe I'll send another group to see Moria has something
  to do with increased Orc activity.
  
  Massing at Osgiliath / Heirs of Numenor (3019 TA) [Sauron: I will
  launch multiple attacks against Gondor on various fronts to see how
  strong their defences are. Also, while they are busy defending my
  'attacks', I will sneak my Nazguls disguised as 'Black Riders' across
  the river Anduin in search for this Shire, Baggins to get my Ring
  back.]