While many hams can get by with the rule of thumb that you want vertical polarization for HF DX antennas at low heights and horizontal polarization at great heights, advanced hams can benefit from learning why this is the case.

• The Effects of Real Ground on Antennas -- Part 1 
  QST February 1984, pp. 15-18
  You've seen free-space antenna patterns, and even pattern calculations that include the effect of a theoretical, perfect ground. But what is your antenna really doing?
  Feedback (QST March 1984):
  Parts of Fig. 3 in this article appear with wrong captions. The plot over caption I belongs with caption K; plot K should be with caption L and plot L with caption I.
• The Effects of Real Ground on Antennas -- Part 2
  QST April 1984, pp. 34-36
• The Effects of Real Ground on Antennas -- Part 3
  QST June 1984, pp. 30-35
• The Effects of Real Ground on Antennas -- Part 4
  QST August 1984, pp. 31-35
• The Effects of Real Ground on Antennas -- Part 5
  QST November 1984, pp. 35-39
• Verticals, Ground Systems and Some History
  QST July 2000, pp. 38-44
  The answers on what makes a vertical antenna work.

Rudy N6LF has done a lot of work radial systems and published his work on Antennas by N6LF.