Who is the king described in 2 Kings 8:1–6? 2 Kings 8:1–6 (ESV)
  1 Now Elisha had said to the woman whose son he had restored to life, “Arise, and depart with your household, and sojourn wherever you can, for the Lord has called for a famine, and it will come upon the land for seven years.” 2 So the woman arose and did according to the word of the man of God. She went with her household and sojourned in the land of the Philistines seven years. 3 And at the end of the seven years, when the woman returned from the land of the Philistines, she went to appeal to the king for her house and her land. 4 Now the king was talking with Gehazi the servant of the man of God, saying, “Tell me all the great things that Elisha has done.” 5 And while he was telling the king how Elisha had restored the dead to life, behold, the woman whose son he had restored to life appealed to the king for her house and her land. And Gehazi said, “My lord, O king, here is the woman, and here is her son whom Elisha restored to life.” 6 And when the king asked the woman, she told him. So the king appointed an official for her, saying, “Restore all that was hers, together with all the produce of the fields from the day that she left the land until now.”


In the story of The Shunammite's Land Restored, we see a king who asks Gehazi to tell great things that Elisha has done. Who is this king? Where else in the Bible he is mentioned? Was he a king of Israel?
This is Jehoram, King of Israel. The woman sojourns to The land of the Philestines to avoid the famine, and then returns to Israel.  

A good exegesis and commentary of this, from the perspective of Gehazi, can be found here:
http://biblehub.com/topical/g/gehazi.htm

More is written about Jehoram starting in 2 Kings 3:


  1 In the eighteenth year of Jehoshaphat king of Judah, Jehoram the son of Ahab became king over Israel in Samaria, and he reigned twelve years.
  2 He did what was evil in the sight of the LORD, though not like his father and mother, for he put away the pillar of Baal that his father had made.
  3 Nevertheless, he clung to the sin of Jeroboam the son of Nebat, which he made Israel to sin; he did not depart from it. - 2 Kings 3:1-3 ESV


It should be noted that the subject of Jehoram is a little confusing,since there were 2 Jehorams, but Jehoram, King of Judah (also known as Joram) did not come into power until after this event.  This confusion between Jehorams and the dating conventions has been addressed very well by Edwin Thiele:
http://www.thischristianjourney.com/thischristianjourney/generalpages/joram_jehoram_problem.htm